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Subject[PATCH v3] KVM: MMU: lazily drop large spte
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 avoid

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.

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
---
Changelog:
v3:
- address Gleb's comments, we make the function return true if flush is
needed instead of returning it via pointer to a variable
- improve the changelog

arch/x86/kvm/mmu.c | 23 +++++++----------------
1 files changed, 7 insertions(+), 16 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 42ba85c..ff2fc80 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1106,8 +1106,7 @@ 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:
@@ -1116,10 +1115,9 @@ 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;

@@ -1129,17 +1127,11 @@ 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,
@@ -1151,11 +1143,8 @@ 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);
}

@@ -2611,6 +2600,8 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t v, int write,
break;
}

+ drop_large_spte(vcpu, iterator.sptep);
+
if (!is_shadow_present_pte(*iterator.sptep)) {
u64 base_addr = iterator.addr;

--
1.7.7.6


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