linux-kernel - Re: [PATCH] emulate accessed bit for EPT

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Date:	Mon, 08 Feb 2010 12:27:24 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Rik van Riel <riel@...hat.com>
CC:	jdike@...toit.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, aarcange@...hat.com,
	mtosatti@...hat.com
Subject: Re: [PATCH] emulate accessed bit for EPT

On 02/03/2010 11:11 PM, Rik van Riel wrote:
> Currently KVM pretends that pages with EPT mappings never got
> accessed.  This has some side effects in the VM, like swapping
> out actively used guest pages and needlessly breaking up actively
> used hugepages.
>
> We can avoid those very costly side effects by emulating the
> accessed bit for EPT PTEs, which should only be slightly costly
> because pages pass through page_referenced infrequently.
>
> TLB flushing is taken care of by kvm_mmu_notifier_clear_flush_young().
>
> This seems to help prevent KVM guests from being swapped out when
> they should not on my system.
>
>    

Applied, thanks.

>
> -	/* always return old for EPT */
> +	/*
> +	 * Emulate the accessed bit for EPT, by checking if this page has
> +	 * an EPT mapping, and clearing it if it does. On the next access,
> +	 * a new EPT mapping will be established.
> +	 * This has some overhead, but not as much as the cost of swapping
> +	 * out actively used pages or breaking up actively used hugepages.
> +	 */
>   	if (!shadow_accessed_mask)
> -		return 0;
> +		return kvm_unmap_rmapp(kvm, rmapp, data);
>    

This could be optimized by using a software-available bit for 'present' 
and the rwx bits for young, that is:

   (present, rwx) -> the page is present and recently accessed, will not 
cause EPT violation
   (present, !rwx) -> page is present but old, will cause EPT violation 
but not rmap games and get_user_pages_fast().

However that's best done later if ever.

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

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