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

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Date:	Wed, 28 Nov 2012 22:59:35 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.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 3/3] KVM: x86: improve reexecute_instruction

On 11/28/2012 10:12 PM, Gleb Natapov wrote:
> On Tue, Nov 27, 2012 at 11:30:24AM +0800, Xiao Guangrong wrote:
>> On 11/27/2012 06:41 AM, Marcelo Tosatti wrote:
>>
>>>>
>>>> -	return false;
>>>> +again:
>>>> +	page_fault_count = ACCESS_ONCE(vcpu->kvm->arch.page_fault_count);
>>>> +
>>>> +	/*
>>>> +	 * if emulation was due to access to shadowed page table
>>>> +	 * and it failed try to unshadow page and re-enter the
>>>> +	 * guest to let CPU execute the instruction.
>>>> +	 */
>>>> +	kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
>>>> +	emulate = vcpu->arch.mmu.page_fault(vcpu, cr3, PFERR_WRITE_MASK, false);
>>>
>>> Can you explain what is the objective here?
>>>
>>
>> Sure. :)
>>
>> The instruction emulation is caused by fault access on cr3. After unprotect
>> the target page, we call vcpu->arch.mmu.page_fault to fix the mapping of cr3.
>> if it return 1, mmu can not fix the mapping, we should report the error,
>> otherwise it is good to return to guest and let it re-execute the instruction
>> again.
>>
>> page_fault_count is used to avoid the race on other vcpus, since after we
>> unprotect the target page, other cpu can enter page fault path and let the
>> page be write-protected again.
>>
>> This way can help us to detect all the case that mmu can not be fixed.
>>
> Can you write this in a comment above vcpu->arch.mmu.page_fault()?

Okay, if Marcelo does not object this way. :)

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