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Subject[PATCH 3/5] KVM: MMU: lazily drop large spte
Date
Currently, kvm zaps the large spte if write-protected is needed, the later
read can fault on that spte. Actually, we can make the large spte readonly
instead of making them un-present, the page fault caused by read access can
be avoided

The idea is from Avi:
| As I mentioned before, write-protecting a large spte is a good idea,
| since it moves some work from protect-time to fault-time, so it reduces
| jitter. This removes the need for the return value.

This version has fixed the issue reported in 6b73a9606, the reason of that
issue is that fast_page_fault() directly sets the readonly large spte to
writable but only dirty the first page into the dirty-bitmap that means
other pages are missed. Fixed it by only the normal sptes (on the
PT_PAGE_TABLE_LEVEL level) can be fast fixed

Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
---
arch/x86/kvm/mmu.c | 34 ++++++++++++++++++----------------
arch/x86/kvm/x86.c | 8 ++++++--
2 files changed, 24 insertions(+), 18 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 046ef10..b8468c8 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1176,8 +1176,7 @@ static void drop_large_spte(struct kvm_vcpu *vcpu, u64 *sptep)

/*
* Write-protect on the specified @sptep, @pt_protect indicates whether
- * spte writ-protection is caused by protecting shadow page table.
- * @flush indicates whether tlb need be flushed.
+ * spte write-protection is caused by protecting shadow page table.
*
* Note: write protection is difference between drity logging and spte
* protection:
@@ -1186,10 +1185,9 @@ static void drop_large_spte(struct kvm_vcpu *vcpu, u64 *sptep)
* - for spte protection, the spte can be writable only after unsync-ing
* shadow page.
*
- * Return true if the spte is dropped.
+ * Return true if tlb need be flushed.
*/
-static bool
-spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush, bool pt_protect)
+static bool spte_write_protect(struct kvm *kvm, u64 *sptep, bool pt_protect)
{
u64 spte = *sptep;

@@ -1199,17 +1197,11 @@ spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush, bool pt_protect)

rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);

- if (__drop_large_spte(kvm, sptep)) {
- *flush |= true;
- return true;
- }
-
if (pt_protect)
spte &= ~SPTE_MMU_WRITEABLE;
spte = spte & ~PT_WRITABLE_MASK;

- *flush |= mmu_spte_update(sptep, spte);
- return false;
+ return mmu_spte_update(sptep, spte);
}

static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,
@@ -1221,11 +1213,8 @@ static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,

for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
BUG_ON(!(*sptep & PT_PRESENT_MASK));
- if (spte_write_protect(kvm, sptep, &flush, pt_protect)) {
- sptep = rmap_get_first(*rmapp, &iter);
- continue;
- }

+ flush |= spte_write_protect(kvm, sptep, pt_protect);
sptep = rmap_get_next(&iter);
}

@@ -2877,6 +2866,19 @@ static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level,
goto exit;

/*
+ * Do not fix write-permission on the large spte since we only dirty
+ * the first page into the dirty-bitmap in fast_pf_fix_direct_spte()
+ * that means other pages are missed if its slot is dirty-logged.
+ *
+ * Instead, we let the slow page fault path create a normal spte to
+ * fix the access.
+ *
+ * See the comments in kvm_arch_commit_memory_region().
+ */
+ if (sp->role.level > PT_PAGE_TABLE_LEVEL)
+ goto exit;
+
+ /*
* Currently, fast page fault only works for direct mapping since
* the gfn is not stable for indirect shadow page.
* See Documentation/virtual/kvm/locking.txt to get more detail.
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d906391..6bcc343 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7301,8 +7301,12 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
/*
* Write protect all pages for dirty logging.
- * Existing largepage mappings are destroyed here and new ones will
- * not be created until the end of the logging.
+ *
+ * All the sptes including the large sptes which point to this
+ * slot are set to readonly. We can not create any new large
+ * spte on this slot until the end of the logging.
+ *
+ * See the comments in fast_page_fault().
*/
if ((change != KVM_MR_DELETE) && (mem->flags & KVM_MEM_LOG_DIRTY_PAGES))
kvm_mmu_slot_remove_write_access(kvm, mem->slot);
--
1.8.1.4


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