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Subject[PATCH 03/12] 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.

[
It has fixed the issue reported in 6b73a9606 by stopping fast page fault
marking the large spte to writable
]

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
---
arch/x86/kvm/mmu.c | 36 +++++++++++++++++-------------------
1 file changed, 17 insertions(+), 19 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index cf163ca..35d4b50 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1181,8 +1181,7 @@ 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:
@@ -1191,10 +1190,9 @@ 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;

@@ -1204,17 +1202,11 @@ 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,
@@ -1226,11 +1218,8 @@ 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);
}

@@ -2701,6 +2690,8 @@ 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;

@@ -2855,7 +2846,7 @@ fast_pf_fix_direct_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
* - false: let the real page fault path to fix it.
*/
static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level,
- u32 error_code)
+ u32 error_code, bool force_pt_level)
{
struct kvm_shadow_walk_iterator iterator;
struct kvm_mmu_page *sp;
@@ -2884,6 +2875,13 @@ static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level,
goto exit;

/*
+ * Can not map the large spte to writable if the page is dirty
+ * logged.
+ */
+ if (sp->role.level > PT_PAGE_TABLE_LEVEL && force_pt_level)
+ goto exit;
+
+ /*
* Check if it is a spurious fault caused by TLB lazily flushed.
*
* Need not check the access of upper level table entries since
@@ -2944,7 +2942,7 @@ static int nonpaging_map(struct kvm_vcpu *vcpu, gva_t v, u32 error_code,
} else
level = PT_PAGE_TABLE_LEVEL;

- if (fast_page_fault(vcpu, v, level, error_code))
+ if (fast_page_fault(vcpu, v, level, error_code, force_pt_level))
return 0;

mmu_seq = vcpu->kvm->mmu_notifier_seq;
@@ -3422,7 +3420,7 @@ static int tdp_page_fault(struct kvm_vcpu *vcpu, gva_t gpa, u32 error_code,
} else
level = PT_PAGE_TABLE_LEVEL;

- if (fast_page_fault(vcpu, gpa, level, error_code))
+ if (fast_page_fault(vcpu, gpa, level, error_code, force_pt_level))
return 0;

mmu_seq = vcpu->kvm->mmu_notifier_seq;
--
1.8.1.4


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