Re: performance with guests running 2.4 kernels (specifically RHEL3)

Avi Kivity
2008-05-11T12:32:04Z

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Avi Kivity wrote:
>
> I asked fo this thinking bypass_guest_pf may help show more
> information. But thinking a bit more, it will not.
>
> I think I do know what the problem is. I will try it out. Is there a
> free clone (like centos) available somewhere?

This patch tracks down emulated accesses to speculated ptes and marks
them as accessed, preventing the flooding on centos-3.1. Unfortunately
it also causes a host oops midway through the boot process.

I believe the oops is merely exposed by the patch, not caused by it.

--
error compiling committee.c: too many arguments to function

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diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 3d769c3..8c1e7f3 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1127,8 +1127,10 @@ unshadowed:
else
kvm_release_pfn_clean(pfn);
}
- if (!ptwrite || !*ptwrite)
+ if (speculative) {
vcpu->arch.last_pte_updated = shadow_pte;
+ vcpu->arch.last_pte_gfn = gfn;
+ }
}

static void nonpaging_new_cr3(struct kvm_vcpu *vcpu)
@@ -1674,6 +1676,17 @@ static void mmu_guess_page_from_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
vcpu->arch.update_pte.pfn = pfn;
}

+static void kvm_mmu_access_page(struct kvm_vcpu *vcpu, gfn_t gfn)
+{
+ u64 *spte = vcpu->arch.last_pte_updated;
+
+ if (spte
+ && vcpu->arch.last_pte_gfn == gfn
+ && shadow_accessed_mask
+ && !(*spte & shadow_accessed_mask))
+ set_bit(PT_ACCESSED_SHIFT, spte);
+}
+
void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
const u8 *new, int bytes)
{
@@ -1697,13 +1710,14 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
pgprintk("%s: gpa %llx bytes %d\n", __func__, gpa, bytes);
mmu_guess_page_from_pte_write(vcpu, gpa, new, bytes);
spin_lock(&vcpu->kvm->mmu_lock);
+ kvm_mmu_access_page(vcpu, gfn);
kvm_mmu_free_some_pages(vcpu);
++vcpu->kvm->stat.mmu_pte_write;
kvm_mmu_audit(vcpu, "pre pte write");
if (gfn == vcpu->arch.last_pt_write_gfn
&& !last_updated_pte_accessed(vcpu)) {
++vcpu->arch.last_pt_write_count;
- if (vcpu->arch.last_pt_write_count >= 3)
+ if (vcpu->arch.last_pt_write_count >= 4)
flooded = 1;
} else {
vcpu->arch.last_pt_write_gfn = gfn;
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 1730757..258e5d5 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -15,7 +15,8 @@
#define PT_USER_MASK (1ULL << 2)
#define PT_PWT_MASK (1ULL << 3)
#define PT_PCD_MASK (1ULL << 4)
-#define PT_ACCESSED_MASK (1ULL << 5)
+#define PT_ACCESSED_SHIFT 5
+#define PT_ACCESSED_MASK (1ULL << PT_ACCESSED_SHIFT)
#define PT_DIRTY_MASK (1ULL << 6)
#define PT_PAGE_SIZE_MASK (1ULL << 7)
#define PT_PAT_MASK (1ULL << 7)
diff --git a/include/asm-x86/kvm_host.h b/include/asm-x86/kvm_host.h
index 1d8cd01..0bdb392 100644
--- a/include/asm-x86/kvm_host.h
+++ b/include/asm-x86/kvm_host.h
@@ -242,6 +242,7 @@ struct kvm_vcpu_arch {
gfn_t last_pt_write_gfn;
int last_pt_write_count;
u64 *last_pte_updated;
+ gfn_t last_pte_gfn;

struct {
gfn_t gfn; /* presumed gfn during guest pte update */

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