linux-kernel - Re: [PATCH v3 1/7] KVM: MMU: fix release noslot pfn

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Message-ID: <20120924120424.GB23096@redhat.com>
Date:	Mon, 24 Sep 2012 14:04:24 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 1/7] KVM: MMU: fix release noslot pfn

On Mon, Sep 24, 2012 at 07:49:37PM +0800, Xiao Guangrong wrote:
> On 09/24/2012 07:24 PM, Gleb Natapov wrote:
> > On Mon, Sep 24, 2012 at 12:59:32PM +0800, Xiao Guangrong wrote:
> >> On 09/23/2012 05:13 PM, Gleb Natapov wrote:
> >>> On Fri, Sep 21, 2012 at 02:57:19PM +0800, Xiao Guangrong wrote:
> >>>> We can not directly call kvm_release_pfn_clean to release the pfn
> >>>> since we can meet noslot pfn which is used to cache mmio info into
> >>>> spte
> >>>>
> >>> Wouldn't it be better to move the check into kvm_release_pfn_clean()?
> >>
> >> I think there is no reason for us to prefer to adding this branch in
> >> the common code. :)
> > 
> > Is the function performance critical? Is function called without the check
> > on a hot path?  The function already contains much heavier kvm_is_mmio_pfn()
> > check. If most/all function invocation require check before call it's
> > better to move it inside.
> 
> It is not most/all functions need do this check - it is only needed on x86 mmu
> page-fault/prefetch path.
At least on x86 there 7 calls to kvm_release_pfn_clean(), 5 of them are
guarded by is_noslot_pfn() (after this patch) and one by even stronger
is_error_pfn(). I guess when/if other architectures will add MMIO MMU
caching they will need to guard kvm_release_pfn_clean() by is_noslot_pfn()
too in most cases. I am not insisting, but as this patch shows it is
easy to miss the check before calling the function.

--
			Gleb.
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