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

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Message-ID: <4E2FD8F7.40303@redhat.com>
Date:	Wed, 27 Jul 2011 12:23:03 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 11/11] KVM: MMU: improve write flooding detected

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

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