linux-kernel - Re: [PATCH 09/13] KVM: MMU: get expected spte out of mmu-lock

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Date:	Sun, 01 Apr 2012 18:53:10 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 09/13] KVM: MMU: get expected spte out of mmu-lock

On 03/29/2012 11:25 AM, Xiao Guangrong wrote:
> It depends on PTE_LIST_WRITE_PROTECT bit in rmap which let us quickly know
> whether the page is writable out of mmu-lock
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> ---
>  arch/x86/kvm/mmu.c         |   17 +++++++++++++----
>  arch/x86/kvm/paging_tmpl.h |    2 +-
>  2 files changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 3887a07..c029185 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1148,6 +1148,12 @@ static int rmap_write_protect(struct kvm *kvm, u64 gfn)
>
>  	*rmapp |= PTE_LIST_WRITE_PROTECT;
>
> +	/*
> +	 * Setting PTE_LIST_WRITE_PROTECT bit before doing page
> +	 * write-protect.
> +	 */
> +	smp_mb();
> +

wmb only needed.

Would it be better to store this bit in all the sptes instead?  We're
touching them in any case.  More work to clear them, but
un-write-protecting a page is beneficial anyway as it can save a fault.


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

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