linux-kernel - Re: [PATCH v2 21/22] KVM: MMU: mmio page fault support

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Message-ID: <4E0B1A86.5000801@cn.fujitsu.com>
Date:	Wed, 29 Jun 2011 20:28:54 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 21/22] KVM: MMU: mmio page fault support

On 06/29/2011 05:22 PM, Avi Kivity wrote:
> On 06/22/2011 05:36 PM, Xiao Guangrong wrote:
>> The idea is from Avi:
>>
>> | We could cache the result of a miss in an spte by using a reserved bit, and
>> | checking the page fault error code (or seeing if we get an ept violation or
>> | ept misconfiguration), so if we get repeated mmio on a page, we don't need to
>> | search the slot list/tree.
>> | (https://lkml.org/lkml/2011/2/22/221)
>>
>> When the page fault is caused by mmio, we cache the info in the shadow page
>> table, and also set the reserved bits in the shadow page table, so if the mmio
>> is caused again, we can quickly identify it and emulate it directly
>>
>> Searching mmio gfn in memslots is heavy since we need to walk all memeslots, it
>> can be reduced by this feature, and also avoid walking guest page table for
>> soft mmu.
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 1319050..e69a47a 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -197,6 +197,41 @@ static u64 __read_mostly shadow_x_mask;    /* mutual exclusive with nx_mask */
>>   static u64 __read_mostly shadow_user_mask;
>>   static u64 __read_mostly shadow_accessed_mask;
>>   static u64 __read_mostly shadow_dirty_mask;
>> +static u64 __read_mostly shadow_mmio_mask = (0xffull<<  49 | 1ULL);
> 
> One bit is shifted out.   And it will fail with 52-bit MAXPHYADDR.
> 
> Please in addition, set the xwr bits to an invalid pattern on EPT (there is an MSR which specifies which patterns are valid; for example execute-only or write-only are invalid).  If all patterns are valid AND MAXPHYADDR == 52, then just set the mask to 0 and it the optimization will be disabled.
> 

OK, will fix, thanks!
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