linux-kernel - Re: [PATCH v3 0/6] KVM: MMU: fast invalidate all mmio sptes

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Message-ID: <20130610170341.GJ29022@redhat.com>
Date:	Mon, 10 Jun 2013 20:03:41 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
Cc:	Xiao Guangrong <xiaoguangrong.eric@...il.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	avi.kivity@...il.com, mtosatti@...hat.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 0/6] KVM: MMU: fast invalidate all mmio sptes

On Mon, Jun 10, 2013 at 10:43:52PM +0900, Takuya Yoshikawa wrote:
> On Mon, 10 Jun 2013 16:39:37 +0800
> Xiao Guangrong <xiaoguangrong.eric@...il.com> wrote:
> 
> > On 06/10/2013 03:56 PM, Gleb Natapov wrote:
> > > On Fri, Jun 07, 2013 at 04:51:22PM +0800, Xiao Guangrong wrote:
> 
> > > Looks good to me, but doesn't tis obsolete kvm_mmu_zap_mmio_sptes() and
> > > sp->mmio_cached, so they should be removed as part of the patch series?
> > 
> > Yes, i agree, they should be removed. :)
> 
> I'm fine with removing it but please make it clear that you all agree
> on the same basis.
> 
> Last time, Paolo mentioned the possibility to use some bits of spte for
> other things.  The suggestion there was to keep sp->mmio_cached code
> for the time we would need to reduce the bits for generation numbers.
> 
> Do you think that zap_all() is now preemptible and can treat the
> situation reasonably well as the current kvm_mmu_zap_mmio_sptes()?
> 
> One downside is the need to zap unrelated shadow pages, but if this case
> is really very rare, yes I agree, it should not be a problem: it depends
> on how many bits we can use.
> 
> Just please reconfirm.
> 
That was me who mention the possibility to use some bits of spte for
other things. But for now I have a use for one bit only. Now that you
have reminded me about that discussion I am not so sure we want to
remove kvm_mmu_zap_mmio_sptes(), but on the other hand it is non
preemptable, so large number of mmio sptes can cause soft lockups.
zap_all() is better in this regards now.

--
			Gleb.
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