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.
 */
static void gru_cb_set_istatus_active(struct gru_instruction_bits *cbk)
{
	if (cbk) {
		cbk->istatus = CBS_ACTIVE;
	}
}

/*
 * 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,
			  struct gru_instruction_bits *cbk)
{
	int pageshift = 0, asid, write, ret, atomic = !cbk;
	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) {
		gru_flush_cache(tfh);
		if (tfh->status != TFHSTATUS_EXCEPTION)
			goto failnoexception;
		STAT(tfh_stale_on_fault);
	}
	if (tfh->state == TFHSTATE_IDLE)
		goto failidle;
	if (tfh->state == TFHSTATE_MISS_FMM && cbk)
		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(cbk);
	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 (!cbk)
		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 (cbk)
		gru_flush_cache(cbk);
	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 (cbk)
		gru_flush_cache(cbk);
	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 (!cbk)
		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);

	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)) {
			gts->ustats.fmm_tlbdropin++;
			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,
			   void *cb)
{
	struct gru_mm_struct *gms = gts->ts_gms;
	int ret;

	gts->ustats.upm_tlbdropin++;
	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;
	void *cbk;
	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;

	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);
		cbk = get_gseg_base_address_cb(gts->ts_gru->gs_gru_base_vaddr,
				gts->ts_ctxnum, ucbnum);
		ret = gru_user_dropin(gts, tfh, cbk);
	}
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);
		gru_flush_cache(cbe);	/* CBE not coherent */
		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;
		gru_flush_cache(cbe);
		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;
	struct gru_mm_struct *gms;

	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;

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

	return 0;
}

/*
 * Fetch GSEG statisticss
 */
long gru_get_gseg_statistics(unsigned long arg)
{
	struct gru_thread_state *gts;
	struct gru_get_gseg_statistics_req req;

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

	/*
	 * The library creates arrays of contexts for threaded programs.
	 * If no gts exists in the array, the context has never been used & all
	 * statistics are implicitly 0.
	 */
	gts = gru_find_lock_gts(req.gseg);
	if (gts) {
		memcpy(&req.stats, &gts->ustats, sizeof(gts->ustats));
		gru_unlock_gts(gts);
	} else {
		memset(&req.stats, 0, sizeof(gts->ustats));
	}

	if (copy_to_user((void __user *)arg, &req, sizeof(req)))
		return -EFAULT;

	return 0;
}

/*
 * Register the current task as the user of the GSEG slice.
 * Needed for TLB fault interrupt targeting.
 */
int gru_set_context_option(unsigned long arg)
{
	struct gru_thread_state *gts;
	struct gru_set_context_option_req req;
	int ret = 0;

	STAT(set_context_option);
	if (copy_from_user(&req, (void __user *)arg, sizeof(req)))
		return -EFAULT;
	gru_dbg(grudev, "op %d, gseg 0x%lx, value1 0x%lx\n", req.op, req.gseg, req.val1);

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

	switch (req.op) {
	case sco_gseg_owner:
 		/* Register the current task as the GSEG owner */
		gts->ts_tgid_owner = current->tgid;
		break;
	case sco_cch_req_slice:
 		/* Set the CCH slice option */
		gts->ts_cch_req_slice = req.val1 & 3;
		break;
	default:
		ret = -EINVAL;
	}
	gru_unlock_gts(gts);

	return ret;
}