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SubjectRe: [RFC] respect the referenced bit of KVM guest pages?
On 08/05/2009 10:58 AM, Avi Kivity wrote:
> On 08/05/2009 05:40 AM, Wu Fengguang wrote:
>> Greetings,
>>
>> Jeff Dike found that many KVM pages are being refaulted in 2.6.29:
>>
>> "Lots of pages between discarded due to memory pressure only to be
>> faulted back in soon after. These pages are nearly all stack pages.
>> This is not consistent - sometimes there are relatively few such pages
>> and they are spread out between processes."
>>
>> The refaults can be drastically reduced by the following patch, which
>> respects the referenced bit of all anonymous pages (including the KVM
>> pages).
>>
>> However it risks reintroducing the problem addressed by commit 7e9cd4842
>> (fix reclaim scalability problem by ignoring the referenced bit,
>> mainly the pte young bit). I wonder if there are better solutions?
>
> How do you distinguish between kvm pages and non-kvm anonymous pages?
> More importantly, why should you?
>
> Jeff, do you see the refaults on Nehalem systems? If so, that's
> likely due to the lack of an accessed bit on EPT pagetables. It would
> be interesting to compare with Barcelona (which does).
>
> If that's indeed the case, we can have the EPT ageing mechanism give
> pages a bit more time around by using an available bit in the EPT PTEs
> to return accessed on the first pass and not-accessed on the second.
>

The attached patch implements this.

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

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 7b53614..310938a 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -195,6 +195,7 @@ static u64 __read_mostly shadow_x_mask; /* mutual exclusive with nx_mask */
static u64 __read_mostly shadow_user_mask;
static u64 __read_mostly shadow_accessed_mask;
static u64 __read_mostly shadow_dirty_mask;
+static int __read_mostly shadow_accessed_shift;

static inline u64 rsvd_bits(int s, int e)
{
@@ -219,6 +220,8 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
{
shadow_user_mask = user_mask;
shadow_accessed_mask = accessed_mask;
+ shadow_accessed_shift
+ = find_first_bit((void *)&shadow_accessed_mask, 64);
shadow_dirty_mask = dirty_mask;
shadow_nx_mask = nx_mask;
shadow_x_mask = x_mask;
@@ -817,11 +820,11 @@ static int kvm_age_rmapp(struct kvm *kvm, unsigned long *rmapp)
while (spte) {
int _young;
u64 _spte = *spte;
- BUG_ON(!(_spte & PT_PRESENT_MASK));
- _young = _spte & PT_ACCESSED_MASK;
+ BUG_ON(!(_spte & shadow_accessed_mask));
+ _young = _spte & shadow_accessed_mask;
if (_young) {
young = 1;
- clear_bit(PT_ACCESSED_SHIFT, (unsigned long *)spte);
+ clear_bit(shadow_accessed_shift, (unsigned long *)spte);
}
spte = rmap_next(kvm, rmapp, spte);
}
@@ -2572,7 +2575,7 @@ static void kvm_mmu_access_page(struct kvm_vcpu *vcpu, gfn_t gfn)
&& shadow_accessed_mask
&& !(*spte & shadow_accessed_mask)
&& is_shadow_present_pte(*spte))
- set_bit(PT_ACCESSED_SHIFT, (unsigned long *)spte);
+ set_bit(shadow_accessed_shift, (unsigned long *)spte);
}

void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 0ba706e..bc99367 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -4029,7 +4029,7 @@ static int __init vmx_init(void)
bypass_guest_pf = 0;
kvm_mmu_set_base_ptes(VMX_EPT_READABLE_MASK |
VMX_EPT_WRITABLE_MASK);
- kvm_mmu_set_mask_ptes(0ull, 0ull, 0ull, 0ull,
+ kvm_mmu_set_mask_ptes(0ull, 1ull << 63, 0ull, 0ull,
VMX_EPT_EXECUTABLE_MASK);
kvm_enable_tdp();
} else
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