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

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Message-ID: <20121127233246.GB8295@amt.cnet>
Date:	Tue, 27 Nov 2012 21:32:46 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	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 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.

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