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

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Message-ID: <50B42FC7.2080805@linux.vnet.ibm.com>
Date:	Tue, 27 Nov 2012 11:13:11 +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 11/27/2012 06:37 AM, Marcelo Tosatti wrote:
> On Tue, Nov 20, 2012 at 07:59:10AM +0800, Xiao Guangrong wrote:
>> Currently, reexecute_instruction refused to retry all instructions. If
>> nested npt is used, the emulation may be caused by shadow page, it can
>> be fixed by dropping the shadow page
>>
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
>> ---
>>  arch/x86/kvm/x86.c |   14 ++++++++------
>>  1 files changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 7be8452..5fe72cc 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -4469,17 +4469,19 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu)
>>  	return r;
>>  }
>>
>> -static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t gva)
>> +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.

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