linux-kernel - Re: [PATCH v4 4/6] KVM: MMU: prefetch ptes when intercepted guest #PF

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Date:	Fri, 2 Jul 2010 13:54:32 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH v4 4/6] KVM: MMU: prefetch ptes when intercepted guest #PF

On Thu, Jul 01, 2010 at 09:55:17PM +0800, Xiao Guangrong wrote:
> Support prefetch ptes when intercept guest #PF, avoid to #PF by later
> access
> 
> If we meet any failure in the prefetch path, we will exit it and
> not try other ptes to avoid become heavy path
> 
> Note: this speculative will mark page become dirty but it not really
> accessed, the same issue is in other speculative paths like invlpg,
> pte write, fortunately, it just affect host memory management. After
> Avi's patchset named "[PATCH v2 1/4] KVM: MMU: Introduce drop_spte()"
> merged, we will easily fix it. Will do it in the future.
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...fujitsu.com>
> ---
>  arch/x86/kvm/mmu.c         |   79 ++++++++++++++++++++++++++++++++++++++++++++
>  arch/x86/kvm/paging_tmpl.h |   76 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 155 insertions(+), 0 deletions(-)
> 

> +	if (pte_prefetch_topup_memory_cache(vcpu))
> +		return;
> +
> +	for (j = 0; i < max; i++, j++) {
> +		pt_element_t gpte;
> +		unsigned pte_access;
> +		u64 *spte = sp->spt + i;
> +		gfn_t gfn;
> +		pfn_t pfn;
> +
> +		if (spte == sptep)
> +			continue;
> +
> +		if (*spte != shadow_trap_nonpresent_pte)
> +			continue;
> +
> +		gpte = gptep[j];
> +
> +		if (is_rsvd_bits_set(vcpu, gpte, PT_PAGE_TABLE_LEVEL))
> +			break;

BTW, doesnt sync_page also lack reserved bit checking? (unrelated to
this patch).

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