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xen: clarify locking used when pinning a pagetable. 

Add some comments explaining the locking and pinning algorithm when
using split pte locks.  Also implement a minor optimisation of not
pinning the PTE when not using split pte locks.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Xen-devel <xen-devel@lists.xensource.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Jeremy Fitzhardinge 2008-08-19 13:32:51 -07:00 committed by Ingo Molnar
parent 63d3a75d6f
commit 11ad93e59d
1 changed files with 35 additions and 6 deletions
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41
arch/x86/xen/mmu.c
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@ -590,8 +590,6 @@ static int pgd_walk(pgd_t *pgd, int (*func)(struct page *, enum pt_level),
pmdidx_limit = 0;
#endif
flush |= (*func)(virt_to_page(pgd), PT_PGD);
for (pgdidx = 0; pgdidx <= pgdidx_limit; pgdidx++) {
pud_t *pud;
@ -637,7 +635,11 @@ static int pgd_walk(pgd_t *pgd, int (*func)(struct page *, enum pt_level),
}
}
}
out:
/* Do the top level last, so that the callbacks can use it as
a cue to do final things like tlb flushes. */
flush |= (*func)(virt_to_page(pgd), PT_PGD);
return flush;
}
@ -691,6 +693,26 @@ static int pin_page(struct page *page, enum pt_level level)
flush = 0;
/*
* We need to hold the pagetable lock between the time
* we make the pagetable RO and when we actually pin
* it. If we don't, then other users may come in and
* attempt to update the pagetable by writing it,
* which will fail because the memory is RO but not
* pinned, so Xen won't do the trap'n'emulate.
*
* If we're using split pte locks, we can't hold the
* entire pagetable's worth of locks during the
* traverse, because we may wrap the preempt count (8
* bits). The solution is to mark RO and pin each PTE
* page while holding the lock. This means the number
* of locks we end up holding is never more than a
* batch size (~32 entries, at present).
*
* If we're not using split pte locks, we needn't pin
* the PTE pages independently, because we're
* protected by the overall pagetable lock.
*/
ptl = NULL;
if (level == PT_PTE)
ptl = lock_pte(page);
@ -699,10 +721,9 @@ static int pin_page(struct page *page, enum pt_level level)
pfn_pte(pfn, PAGE_KERNEL_RO),
level == PT_PGD ? UVMF_TLB_FLUSH : 0);
if (level == PT_PTE)
if (ptl) {
xen_do_pin(MMUEXT_PIN_L1_TABLE, pfn);
if (ptl) {
/* Queue a deferred unlock for when this batch
is completed. */
xen_mc_callback(do_unlock, ptl);
@ -796,10 +817,18 @@ static int unpin_page(struct page *page, enum pt_level level)
spinlock_t *ptl = NULL;
struct multicall_space mcs;
/*
* Do the converse to pin_page. If we're using split
* pte locks, we must be holding the lock for while
* the pte page is unpinned but still RO to prevent
* concurrent updates from seeing it in this
* partially-pinned state.
*/
if (level == PT_PTE) {
ptl = lock_pte(page);
xen_do_pin(MMUEXT_UNPIN_TABLE, pfn);
if (ptl)
xen_do_pin(MMUEXT_UNPIN_TABLE, pfn);
}
mcs = __xen_mc_entry(0);
@ -837,7 +866,7 @@ static void xen_pgd_unpin(pgd_t *pgd)
#ifdef CONFIG_X86_PAE
/* Need to make sure unshared kernel PMD is unpinned */
pin_page(virt_to_page(pgd_page(pgd[pgd_index(TASK_SIZE)])), PT_PMD);
unpin_page(virt_to_page(pgd_page(pgd[pgd_index(TASK_SIZE)])), PT_PMD);
#endif
pgd_walk(pgd, unpin_page, USER_LIMIT);