linux-kernel - Re: [PATCH 2/3] KVM: x86: let reexecute

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Message-ID: <20121128140155.GH928@redhat.com>
Date:	Wed, 28 Nov 2012 16:01:55 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Marcelo Tosatti <mtosatti@...hat.com>, Avi Kivity <avi@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/3] KVM: x86: let reexecute_instruction work for tdp

On Wed, Nov 28, 2012 at 11:15:13AM +0800, Xiao Guangrong wrote:
> On 11/28/2012 07:32 AM, Marcelo Tosatti wrote:
> > On Tue, Nov 27, 2012 at 11:13:11AM +0800, Xiao Guangrong wrote:
> >>>> +static bool reexecute_instruction(struct kvm_vcpu *vcpu, unsigned long cr2)
> >>>>  {
> >>>> -	gpa_t gpa;
> >>>> +	gpa_t gpa = cr2;
> >>>>  	pfn_t pfn;
> >>>>
> >>>> -	if (tdp_enabled)
> >>>> +	if (!ACCESS_ONCE(vcpu->kvm->arch.indirect_shadow_pages))
> >>>>  		return false;
> >>>
> >>> How is indirect_shadow_pages protected? Why is ACCESS_ONCE() being used
> >>> to read it?
> >>
> >> Hi Marcelo,
> >>
> >> It is protected by mmu-lock for it only be changed when mmu-lock is hold. And
> >> ACCESS_ONCE is used on read path avoiding magic optimization from compiler.
> > 
> > Please switch to mmu_lock protection, there is no reason to have access
> > to this variable locklessly - not performance critical.
> > 
> > For example, there is no use of barriers when modifying the variable.
> 
> This is not bad, the worst case is, the direct mmu failed to unprotect the shadow
> pages, (meet indirect_shadow_pages = 0, but there has shadow pages being shadowed.),
> after enter to guest, we will go into reexecute_instruction again, then it will
> remove shadow pages.
> 
Isn't the same scenario can happen even with mmu lock around
indirect_shadow_pages access?

> But, i do not have strong opinion on it, i respect your idea! :)
> 
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--
			Gleb.
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