linux-kernel - Re: [PATCH] KVM: MMU: optimize for set

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Message-ID: <50C00336.8010307@linux.vnet.ibm.com>
Date:	Thu, 06 Dec 2012 10:30:14 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, Gleb Natapov <gleb@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: MMU: optimize for set_spte

On 12/06/2012 05:09 AM, Marcelo Tosatti wrote:
> On Tue, Dec 04, 2012 at 07:17:11AM +0800, Xiao Guangrong wrote:
>> There are two cases we need to adjust page size in set_spte:
>> 1): the one is other vcpu creates new sp in the window between mapping_level()
>>     and acquiring mmu-lock.
>> 2): the another case is the new sp is created by itself (page-fault path) when
>>     guest uses the target gfn as its page table.
>>
>> In current code, set_spte drop the spte and emulate the access for these case,
>> it works not good:
>> - for the case 1, it may destroy the mapping established by other vcpu, and
>>   do expensive instruction emulation.
>> - for the case 2, it may emulate the access even if the guest is accessing
>>   the page which not used as page table. There is a example, 0~2M is used as
>>   huge page in guest, in this huge page, only page 3 used as page table, then
>>   guest read/writes on other pages can cause instruction emulation.
>>
>> Both of these cases can be fixed by allowing guest to retry the access, it
>> will refault, then we can establish the mapping by using small page
>>
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
>> ---
>>  arch/x86/kvm/mmu.c |   16 ++++++++++++----
>>  1 files changed, 12 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index b875a9e..01d7c2a 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -2382,12 +2382,20 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>>  	    || (!vcpu->arch.mmu.direct_map && write_fault
>>  		&& !is_write_protection(vcpu) && !user_fault)) {
>>
>> +		/*
>> +		 * There are two cases:
>> +		 * - the one is other vcpu creates new sp in the window
>> +		 *   between mapping_level() and acquiring mmu-lock.
>> +		 * - the another case is the new sp is created by itself
>> +		 *   (page-fault path) when guest uses the target gfn as
>> +		 *   its page table.
>> +		 * Both of these cases can be fixed by allowing guest to
>> +		 * retry the access, it will refault, then we can establish
>> +		 * the mapping by using small page.
>> +		 */
>>  		if (level > PT_PAGE_TABLE_LEVEL &&
>> -		    has_wrprotected_page(vcpu->kvm, gfn, level)) {
>> -			ret = 1;
>> -			drop_spte(vcpu->kvm, sptep);
>> +		    has_wrprotected_page(vcpu->kvm, gfn, level))
>>  			goto done;
>> -		}
>>
>>  		spte |= PT_WRITABLE_MASK | SPTE_MMU_WRITEABLE;
>>
>> -- 
>> 1.7.7.6
> 
> ACK.
> 
> Does it fix your testcase?

No.
I will post the new version patch of improving reexecute_instruction.



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