gru: support instruction completion interrupts

[safe/jmp/linux-2.6] / drivers / misc / sgi-gru / grufault.c

/*
 * SN Platform GRU Driver
 *
 *              FAULT HANDLER FOR GRU DETECTED TLB MISSES
 *
 * This file contains code that handles TLB misses within the GRU.
 * These misses are reported either via interrupts or user polling of
 * the user CB.
 *
 *  Copyright (c) 2008 Silicon Graphics, Inc.  All Rights Reserved.
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA
 */

#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/spinlock.h>
#include <linux/mm.h>
#include <linux/hugetlb.h>
#include <linux/device.h>
#include <linux/io.h>
#include <linux/uaccess.h>
#include <linux/security.h>
#include <asm/pgtable.h>
#include "gru.h"
#include "grutables.h"
#include "grulib.h"
#include "gru_instructions.h"
#include <asm/uv/uv_hub.h>

/*
 * Test if a physical address is a valid GRU GSEG address
 */
static inline int is_gru_paddr(unsigned long paddr)
{
        return paddr >= gru_start_paddr && paddr < gru_end_paddr;
}

/*
 * Find the vma of a GRU segment. Caller must hold mmap_sem.
 */
struct vm_area_struct *gru_find_vma(unsigned long vaddr)
{
        struct vm_area_struct *vma;

        vma = find_vma(current->mm, vaddr);
        if (vma && vma->vm_start <= vaddr && vma->vm_ops == &gru_vm_ops)
                return vma;
        return NULL;
}

/*
 * Find and lock the gts that contains the specified user vaddr.
 *
 * Returns:
 *      - *gts with the mmap_sem locked for read and the GTS locked.
 *      - NULL if vaddr invalid OR is not a valid GSEG vaddr.
 */

static struct gru_thread_state *gru_find_lock_gts(unsigned long vaddr)
{
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *vma;
        struct gru_thread_state *gts = NULL;

        down_read(&mm->mmap_sem);
        vma = gru_find_vma(vaddr);
        if (vma)
                gts = gru_find_thread_state(vma, TSID(vaddr, vma));
        if (gts)
                mutex_lock(&gts->ts_ctxlock);
        else
                up_read(&mm->mmap_sem);
        return gts;
}

static struct gru_thread_state *gru_alloc_locked_gts(unsigned long vaddr)
{
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *vma;
        struct gru_thread_state *gts = NULL;

        down_write(&mm->mmap_sem);
        vma = gru_find_vma(vaddr);
        if (vma)
                gts = gru_alloc_thread_state(vma, TSID(vaddr, vma));
        if (gts) {
                mutex_lock(&gts->ts_ctxlock);
                downgrade_write(&mm->mmap_sem);
        } else {
                up_write(&mm->mmap_sem);
        }

        return gts;
}

/*
 * Unlock a GTS that was previously locked with gru_find_lock_gts().
 */
static void gru_unlock_gts(struct gru_thread_state *gts)
{
        mutex_unlock(&gts->ts_ctxlock);
        up_read(&current->mm->mmap_sem);
}

/*
 * Set a CB.istatus to active using a user virtual address. This must be done
 * just prior to a TFH RESTART. The new cb.istatus is an in-cache status ONLY.
 * If the line is evicted, the status may be lost. The in-cache update
 * is necessary to prevent the user from seeing a stale cb.istatus that will
 * change as soon as the TFH restart is complete. Races may cause an
 * occasional failure to clear the cb.istatus, but that is ok.
 *
 * If the cb address is not valid (should not happen, but...), nothing
 * bad will happen.. The get_user()/put_user() will fail but there
 * are no bad side-effects.
 */
static void gru_cb_set_istatus_active(unsigned long __user *cb)
{
        union {
                struct gru_instruction_bits bits;
                unsigned long dw;
        } u;

        if (cb) {
                get_user(u.dw, cb);
                u.bits.istatus = CBS_ACTIVE;
                put_user(u.dw, cb);
        }
}

/*
 * Convert a interrupt IRQ to a pointer to the GRU GTS that caused the
 * interrupt. Interrupts are always sent to a cpu on the blade that contains the
 * GRU (except for headless blades which are not currently supported). A blade
 * has N grus; a block of N consecutive IRQs is assigned to the GRUs. The IRQ
 * number uniquely identifies the GRU chiplet on the local blade that caused the
 * interrupt. Always called in interrupt context.
 */
static inline struct gru_state *irq_to_gru(int irq)
{
        return &gru_base[uv_numa_blade_id()]->bs_grus[irq - IRQ_GRU];
}

/*
 * Read & clear a TFM
 *
 * The GRU has an array of fault maps. A map is private to a cpu
 * Only one cpu will be accessing a cpu's fault map.
 *
 * This function scans the cpu-private fault map & clears all bits that
 * are set. The function returns a bitmap that indicates the bits that
 * were cleared. Note that sense the maps may be updated asynchronously by
 * the GRU, atomic operations must be used to clear bits.
 */
static void get_clear_fault_map(struct gru_state *gru,
                                struct gru_tlb_fault_map *imap,
                                struct gru_tlb_fault_map *dmap)
{
        unsigned long i, k;
        struct gru_tlb_fault_map *tfm;

        tfm = get_tfm_for_cpu(gru, gru_cpu_fault_map_id());
        prefetchw(tfm);         /* Helps on hardware, required for emulator */
        for (i = 0; i < BITS_TO_LONGS(GRU_NUM_CBE); i++) {
                k = tfm->fault_bits[i];
                if (k)
                        k = xchg(&tfm->fault_bits[i], 0UL);
                imap->fault_bits[i] = k;
                k = tfm->done_bits[i];
                if (k)
                        k = xchg(&tfm->done_bits[i], 0UL);
                dmap->fault_bits[i] = k;
        }

        /*
         * Not functionally required but helps performance. (Required
         * on emulator)
         */
        gru_flush_cache(tfm);
}

/*
 * Atomic (interrupt context) & non-atomic (user context) functions to
 * convert a vaddr into a physical address. The size of the page
 * is returned in pageshift.
 *      returns:
 *                0 - successful
 *              < 0 - error code
 *                1 - (atomic only) try again in non-atomic context
 */
static int non_atomic_pte_lookup(struct vm_area_struct *vma,
                                 unsigned long vaddr, int write,
                                 unsigned long *paddr, int *pageshift)
{
        struct page *page;

        /* ZZZ Need to handle HUGE pages */
        if (is_vm_hugetlb_page(vma))
                return -EFAULT;
        *pageshift = PAGE_SHIFT;
        if (get_user_pages
            (current, current->mm, vaddr, 1, write, 0, &page, NULL) <= 0)
                return -EFAULT;
        *paddr = page_to_phys(page);
        put_page(page);
        return 0;
}

/*
 * atomic_pte_lookup
 *
 * Convert a user virtual address to a physical address
 * Only supports Intel large pages (2MB only) on x86_64.
 *      ZZZ - hugepage support is incomplete
 *
 * NOTE: mmap_sem is already held on entry to this function. This
 * guarantees existence of the page tables.
 */
static int atomic_pte_lookup(struct vm_area_struct *vma, unsigned long vaddr,
        int write, unsigned long *paddr, int *pageshift)
{
        pgd_t *pgdp;
        pmd_t *pmdp;
        pud_t *pudp;
        pte_t pte;

        pgdp = pgd_offset(vma->vm_mm, vaddr);
        if (unlikely(pgd_none(*pgdp)))
                goto err;

        pudp = pud_offset(pgdp, vaddr);
        if (unlikely(pud_none(*pudp)))
                goto err;

        pmdp = pmd_offset(pudp, vaddr);
        if (unlikely(pmd_none(*pmdp)))
                goto err;
#ifdef CONFIG_X86_64
        if (unlikely(pmd_large(*pmdp)))
                pte = *(pte_t *) pmdp;
        else
#endif
                pte = *pte_offset_kernel(pmdp, vaddr);

        if (unlikely(!pte_present(pte) ||
                     (write && (!pte_write(pte) || !pte_dirty(pte)))))
                return 1;

        *paddr = pte_pfn(pte) << PAGE_SHIFT;
#ifdef CONFIG_HUGETLB_PAGE
        *pageshift = is_vm_hugetlb_page(vma) ? HPAGE_SHIFT : PAGE_SHIFT;
#else
        *pageshift = PAGE_SHIFT;
#endif
        return 0;

err:
        local_irq_enable();
        return 1;
}

static int gru_vtop(struct gru_thread_state *gts, unsigned long vaddr,
                    int write, int atomic, unsigned long *gpa, int *pageshift)
{
        struct mm_struct *mm = gts->ts_mm;
        struct vm_area_struct *vma;
        unsigned long paddr;
        int ret, ps;

        vma = find_vma(mm, vaddr);
        if (!vma)
                goto inval;

        /*
         * Atomic lookup is faster & usually works even if called in non-atomic
         * context.
         */
        rmb();  /* Must/check ms_range_active before loading PTEs */
        ret = atomic_pte_lookup(vma, vaddr, write, &paddr, &ps);
        if (ret) {
                if (atomic)
                        goto upm;
                if (non_atomic_pte_lookup(vma, vaddr, write, &paddr, &ps))
                        goto inval;
        }
        if (is_gru_paddr(paddr))
                goto inval;
        paddr = paddr & ~((1UL << ps) - 1);
        *gpa = uv_soc_phys_ram_to_gpa(paddr);
        *pageshift = ps;
        return 0;

inval:
        return -1;
upm:
        return -2;
}


/*
 * Drop a TLB entry into the GRU. The fault is described by info in an TFH.
 *      Input:
 *              cb    Address of user CBR. Null if not running in user context
 *      Return:
 *                0 = dropin, exception, or switch to UPM successful
 *                1 = range invalidate active
 *              < 0 = error code
 *
 */
static int gru_try_dropin(struct gru_thread_state *gts,
                          struct gru_tlb_fault_handle *tfh,
                          unsigned long __user *cb)
{
        int pageshift = 0, asid, write, ret, atomic = !cb;
        unsigned long gpa = 0, vaddr = 0;

        /*
         * NOTE: The GRU contains magic hardware that eliminates races between
         * TLB invalidates and TLB dropins. If an invalidate occurs
         * in the window between reading the TFH and the subsequent TLB dropin,
         * the dropin is ignored. This eliminates the need for additional locks.
         */

        /*
         * Error if TFH state is IDLE or FMM mode & the user issuing a UPM call.
         * Might be a hardware race OR a stupid user. Ignore FMM because FMM
         * is a transient state.
         */
        if (tfh->status != TFHSTATUS_EXCEPTION)
                goto failnoexception;
        if (tfh->state == TFHSTATE_IDLE)
                goto failidle;
        if (tfh->state == TFHSTATE_MISS_FMM && cb)
                goto failfmm;

        write = (tfh->cause & TFHCAUSE_TLB_MOD) != 0;
        vaddr = tfh->missvaddr;
        asid = tfh->missasid;
        if (asid == 0)
                goto failnoasid;

        rmb();  /* TFH must be cache resident before reading ms_range_active */

        /*
         * TFH is cache resident - at least briefly. Fail the dropin
         * if a range invalidate is active.
         */
        if (atomic_read(&gts->ts_gms->ms_range_active))
                goto failactive;

        ret = gru_vtop(gts, vaddr, write, atomic, &gpa, &pageshift);
        if (ret == -1)
                goto failinval;
        if (ret == -2)
                goto failupm;

        if (!(gts->ts_sizeavail & GRU_SIZEAVAIL(pageshift))) {
                gts->ts_sizeavail |= GRU_SIZEAVAIL(pageshift);
                if (atomic || !gru_update_cch(gts, 0)) {
                        gts->ts_force_cch_reload = 1;
                        goto failupm;
                }
        }
        gru_cb_set_istatus_active(cb);
        tfh_write_restart(tfh, gpa, GAA_RAM, vaddr, asid, write,
                          GRU_PAGESIZE(pageshift));
        STAT(tlb_dropin);
        gru_dbg(grudev,
                "%s: tfh 0x%p, vaddr 0x%lx, asid 0x%x, ps %d, gpa 0x%lx\n",
                ret ? "non-atomic" : "atomic", tfh, vaddr, asid,
                pageshift, gpa);
        return 0;

failnoasid:
        /* No asid (delayed unload). */
        STAT(tlb_dropin_fail_no_asid);
        gru_dbg(grudev, "FAILED no_asid tfh: 0x%p, vaddr 0x%lx\n", tfh, vaddr);
        if (!cb)
                tfh_user_polling_mode(tfh);
        else
                gru_flush_cache(tfh);
        return -EAGAIN;

failupm:
        /* Atomic failure switch CBR to UPM */
        tfh_user_polling_mode(tfh);
        STAT(tlb_dropin_fail_upm);
        gru_dbg(grudev, "FAILED upm tfh: 0x%p, vaddr 0x%lx\n", tfh, vaddr);
        return 1;

failfmm:
        /* FMM state on UPM call */
        gru_flush_cache(tfh);
        STAT(tlb_dropin_fail_fmm);
        gru_dbg(grudev, "FAILED fmm tfh: 0x%p, state %d\n", tfh, tfh->state);
        return 0;

failnoexception:
        /* TFH status did not show exception pending */
        gru_flush_cache(tfh);
        if (cb)
                gru_flush_cache(cb);
        STAT(tlb_dropin_fail_no_exception);
        gru_dbg(grudev, "FAILED non-exception tfh: 0x%p, status %d, state %d\n", tfh, tfh->status, tfh->state);
        return 0;

failidle:
        /* TFH state was idle  - no miss pending */
        gru_flush_cache(tfh);
        if (cb)
                gru_flush_cache(cb);
        STAT(tlb_dropin_fail_idle);
        gru_dbg(grudev, "FAILED idle tfh: 0x%p, state %d\n", tfh, tfh->state);
        return 0;

failinval:
        /* All errors (atomic & non-atomic) switch CBR to EXCEPTION state */
        tfh_exception(tfh);
        STAT(tlb_dropin_fail_invalid);
        gru_dbg(grudev, "FAILED inval tfh: 0x%p, vaddr 0x%lx\n", tfh, vaddr);
        return -EFAULT;

failactive:
        /* Range invalidate active. Switch to UPM iff atomic */
        if (!cb)
                tfh_user_polling_mode(tfh);
        else
                gru_flush_cache(tfh);
        STAT(tlb_dropin_fail_range_active);
        gru_dbg(grudev, "FAILED range active: tfh 0x%p, vaddr 0x%lx\n",
                tfh, vaddr);
        return 1;
}

/*
 * Process an external interrupt from the GRU. This interrupt is
 * caused by a TLB miss.
 * Note that this is the interrupt handler that is registered with linux
 * interrupt handlers.
 */
irqreturn_t gru_intr(int irq, void *dev_id)
{
        struct gru_state *gru;
        struct gru_tlb_fault_map imap, dmap;
        struct gru_thread_state *gts;
        struct gru_tlb_fault_handle *tfh = NULL;
        int cbrnum, ctxnum;

        STAT(intr);

        gru = irq_to_gru(irq);
        if (!gru) {
                dev_err(grudev, "GRU: invalid interrupt: cpu %d, irq %d\n",
                        raw_smp_processor_id(), irq);
                return IRQ_NONE;
        }
        get_clear_fault_map(gru, &imap, &dmap);
        gru_dbg(grudev,
                "irq %d, gid %d, imap %016lx %016lx, dmap %016lx %016lx\n",
                irq, gru->gs_gid, dmap.fault_bits[0], dmap.fault_bits[1],
                dmap.fault_bits[0], dmap.fault_bits[1]);

        for_each_cbr_in_tfm(cbrnum, dmap.fault_bits) {
                complete(gru->gs_blade->bs_async_wq);
                gru_dbg(grudev, "gid %d, cbr_done %d, done %d\n",
                        gru->gs_gid, cbrnum, gru->gs_blade->bs_async_wq->done);
        }

        for_each_cbr_in_tfm(cbrnum, imap.fault_bits) {
                tfh = get_tfh_by_index(gru, cbrnum);
                prefetchw(tfh); /* Helps on hdw, required for emulator */

                /*
                 * When hardware sets a bit in the faultmap, it implicitly
                 * locks the GRU context so that it cannot be unloaded.
                 * The gts cannot change until a TFH start/writestart command
                 * is issued.
                 */
                ctxnum = tfh->ctxnum;
                gts = gru->gs_gts[ctxnum];

                /*
                 * This is running in interrupt context. Trylock the mmap_sem.
                 * If it fails, retry the fault in user context.
                 */
                if (!gts->ts_force_cch_reload &&
                                        down_read_trylock(&gts->ts_mm->mmap_sem)) {
                        gru_try_dropin(gts, tfh, NULL);
                        up_read(&gts->ts_mm->mmap_sem);
                } else {
                        tfh_user_polling_mode(tfh);
                        STAT(intr_mm_lock_failed);
                }
        }
        return IRQ_HANDLED;
}


static int gru_user_dropin(struct gru_thread_state *gts,
                           struct gru_tlb_fault_handle *tfh,
                           unsigned long __user *cb)
{
        struct gru_mm_struct *gms = gts->ts_gms;
        int ret;

        while (1) {
                wait_event(gms->ms_wait_queue,
                           atomic_read(&gms->ms_range_active) == 0);
                prefetchw(tfh); /* Helps on hdw, required for emulator */
                ret = gru_try_dropin(gts, tfh, cb);
                if (ret <= 0)
                        return ret;
                STAT(call_os_wait_queue);
        }
}

/*
 * This interface is called as a result of a user detecting a "call OS" bit
 * in a user CB. Normally means that a TLB fault has occurred.
 *      cb - user virtual address of the CB
 */
int gru_handle_user_call_os(unsigned long cb)
{
        struct gru_tlb_fault_handle *tfh;
        struct gru_thread_state *gts;
        unsigned long __user *cbp;
        int ucbnum, cbrnum, ret = -EINVAL;

        STAT(call_os);
        gru_dbg(grudev, "address 0x%lx\n", cb);

        /* sanity check the cb pointer */
        ucbnum = get_cb_number((void *)cb);
        if ((cb & (GRU_HANDLE_STRIDE - 1)) || ucbnum >= GRU_NUM_CB)
                return -EINVAL;
        cbp = (unsigned long *)cb;

        gts = gru_find_lock_gts(cb);
        if (!gts)
                return -EINVAL;

        if (ucbnum >= gts->ts_cbr_au_count * GRU_CBR_AU_SIZE)
                goto exit;

        /*
         * If force_unload is set, the UPM TLB fault is phony. The task
         * has migrated to another node and the GSEG must be moved. Just
         * unload the context. The task will page fault and assign a new
         * context.
         */
        if (gts->ts_tgid_owner == current->tgid && gts->ts_blade >= 0 &&
                                gts->ts_blade != uv_numa_blade_id()) {
                STAT(call_os_offnode_reference);
                gts->ts_force_unload = 1;
        }

        /*
         * CCH may contain stale data if ts_force_cch_reload is set.
         */
        if (gts->ts_gru && gts->ts_force_cch_reload) {
                gts->ts_force_cch_reload = 0;
                gru_update_cch(gts, 0);
        }

        ret = -EAGAIN;
        cbrnum = thread_cbr_number(gts, ucbnum);
        if (gts->ts_force_unload) {
                gru_unload_context(gts, 1);
        } else if (gts->ts_gru) {
                tfh = get_tfh_by_index(gts->ts_gru, cbrnum);
                ret = gru_user_dropin(gts, tfh, cbp);
        }
exit:
        gru_unlock_gts(gts);
        return ret;
}

/*
 * Fetch the exception detail information for a CB that terminated with
 * an exception.
 */
int gru_get_exception_detail(unsigned long arg)
{
        struct control_block_extended_exc_detail excdet;
        struct gru_control_block_extended *cbe;
        struct gru_thread_state *gts;
        int ucbnum, cbrnum, ret;

        STAT(user_exception);
        if (copy_from_user(&excdet, (void __user *)arg, sizeof(excdet)))
                return -EFAULT;

        gru_dbg(grudev, "address 0x%lx\n", excdet.cb);
        gts = gru_find_lock_gts(excdet.cb);
        if (!gts)
                return -EINVAL;

        ucbnum = get_cb_number((void *)excdet.cb);
        if (ucbnum >= gts->ts_cbr_au_count * GRU_CBR_AU_SIZE) {
                ret = -EINVAL;
        } else if (gts->ts_gru) {
                cbrnum = thread_cbr_number(gts, ucbnum);
                cbe = get_cbe_by_index(gts->ts_gru, cbrnum);
                prefetchw(cbe);/* Harmless on hardware, required for emulator */
                excdet.opc = cbe->opccpy;
                excdet.exopc = cbe->exopccpy;
                excdet.ecause = cbe->ecause;
                excdet.exceptdet0 = cbe->idef1upd;
                excdet.exceptdet1 = cbe->idef3upd;
                excdet.cbrstate = cbe->cbrstate;
                excdet.cbrexecstatus = cbe->cbrexecstatus;
                ret = 0;
        } else {
                ret = -EAGAIN;
        }
        gru_unlock_gts(gts);

        gru_dbg(grudev,
                "cb 0x%lx, op %d, exopc %d, cbrstate %d, cbrexecstatus 0x%x, ecause 0x%x, "
                "exdet0 0x%lx, exdet1 0x%x\n",
                excdet.cb, excdet.opc, excdet.exopc, excdet.cbrstate, excdet.cbrexecstatus,
                excdet.ecause, excdet.exceptdet0, excdet.exceptdet1);
        if (!ret && copy_to_user((void __user *)arg, &excdet, sizeof(excdet)))
                ret = -EFAULT;
        return ret;
}

/*
 * User request to unload a context. Content is saved for possible reload.
 */
static int gru_unload_all_contexts(void)
{
        struct gru_thread_state *gts;
        struct gru_state *gru;
        int gid, ctxnum;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        foreach_gid(gid) {
                gru = GID_TO_GRU(gid);
                spin_lock(&gru->gs_lock);
                for (ctxnum = 0; ctxnum < GRU_NUM_CCH; ctxnum++) {
                        gts = gru->gs_gts[ctxnum];
                        if (gts && mutex_trylock(&gts->ts_ctxlock)) {
                                spin_unlock(&gru->gs_lock);
                                gru_unload_context(gts, 1);
                                mutex_unlock(&gts->ts_ctxlock);
                                spin_lock(&gru->gs_lock);
                        }
                }
                spin_unlock(&gru->gs_lock);
        }
        return 0;
}

int gru_user_unload_context(unsigned long arg)
{
        struct gru_thread_state *gts;
        struct gru_unload_context_req req;

        STAT(user_unload_context);
        if (copy_from_user(&req, (void __user *)arg, sizeof(req)))
                return -EFAULT;

        gru_dbg(grudev, "gseg 0x%lx\n", req.gseg);

        if (!req.gseg)
                return gru_unload_all_contexts();

        gts = gru_find_lock_gts(req.gseg);
        if (!gts)
                return -EINVAL;

        if (gts->ts_gru)
                gru_unload_context(gts, 1);
        gru_unlock_gts(gts);

        return 0;
}

/*
 * User request to flush a range of virtual addresses from the GRU TLB
 * (Mainly for testing).
 */
int gru_user_flush_tlb(unsigned long arg)
{
        struct gru_thread_state *gts;
        struct gru_flush_tlb_req req;

        STAT(user_flush_tlb);
        if (copy_from_user(&req, (void __user *)arg, sizeof(req)))
                return -EFAULT;

        gru_dbg(grudev, "gseg 0x%lx, vaddr 0x%lx, len 0x%lx\n", req.gseg,
                req.vaddr, req.len);

        gts = gru_find_lock_gts(req.gseg);
        if (!gts)
                return -EINVAL;

        gru_flush_tlb_range(gts->ts_gms, req.vaddr, req.len);
        gru_unlock_gts(gts);

        return 0;
}

/*
 * Register the current task as the user of the GSEG slice.
 * Needed for TLB fault interrupt targeting.
 */
int gru_set_task_slice(long address)
{
        struct gru_thread_state *gts;

        STAT(set_task_slice);
        gru_dbg(grudev, "address 0x%lx\n", address);
        gts = gru_alloc_locked_gts(address);
        if (!gts)
                return -EINVAL;

        gts->ts_tgid_owner = current->tgid;
        gru_unlock_gts(gts);

        return 0;
}