Re: [PATCH 11/11] KVM: MMU: improve write flooding detected

Avi Kivity
2011-07-27T09:23:09Z

On 07/26/2011 02:32 PM, Xiao Guangrong wrote:
> Detecting write-flooding does not work well, when we handle page written, if
> the last speculative spte is not accessed, we treat the page is
> write-flooding, however, we can speculative spte on many path, such as pte
> prefetch, page synced, that means the last speculative spte may be not point
> to the written page and the written page can be accessed via other sptes, so
> depends on the Accessed bit of the last speculative spte is not enough
>
> Instead of detected page accessed, we can detect whether the spte is accessed
> or not, if the spte is not accessed but it is written frequently, we treat is
> not a page table or it not used for a long time
>
> static int get_free_pte_list_desc_nr(struct kvm_vcpu *vcpu)
> {
> struct kvm_mmu_memory_cache *cache;
> @@ -3565,22 +3547,14 @@ static u64 mmu_pte_write_fetch_gpte(struct kvm_vcpu *vcpu, gpa_t *gpa,
> * If we're seeing too many writes to a page, it may no longer be a page table,
> * or we may be forking, in which case it is better to unmap the page.
> */
> -static bool detect_write_flooding(struct kvm_vcpu *vcpu, gfn_t gfn)
> +static bool detect_write_flooding(struct kvm_mmu_page *sp, u64 *spte)
> {
> - bool flooded = false;
> -
> - 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)
> - flooded = true;
> - } else {
> - vcpu->arch.last_pt_write_gfn = gfn;
> - vcpu->arch.last_pt_write_count = 1;
> - vcpu->arch.last_pte_updated = NULL;
> - }
> + if (spte&& !(*spte& shadow_accessed_mask))
> + sp->write_flooding_count++;
> + else
> + sp->write_flooding_count = 0;
>
> - return flooded;
> + return sp->write_flooding_count>= 3;
> }

I think this is a little dangerous. A guest kernel may be instantiating
multiple gptes on a page fault, but guest userspace hits only one of
them (the one which caused the page fault) - I think Windows does this,
but I'm not sure.

Maybe we should inspect parent_ptes instead?

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