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

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Date:	Wed, 28 Nov 2012 11:15:13 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.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 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.

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

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