Re: [PATCH 03/11] KVM: x86: fast emulate repeat string write instructions

Xiao Guangrong
2011-07-27T06:30:45Z

On 07/27/2011 12:26 PM, Gleb Natapov wrote:
> On Wed, Jul 27, 2011 at 09:47:52AM +0800, Xiao Guangrong wrote:
>> On 07/26/2011 08:27 PM, Gleb Natapov wrote:
>>> On Tue, Jul 26, 2011 at 07:26:46PM +0800, Xiao Guangrong wrote:
>>>> We usually use repeat string instructions to clear the page, for example,
>>> By "we" do you mean Linux guest?
>>>
>>
>> I do not know other guests except linux, but, generally rep instruction is
>> not used to update a page table which is been using.
>>
>>>> we call memset to clear a page table, stosb is used in this function, and
>>>> repeated for 1024 times, that means we should occupy mmu lock for 1024 times
>>>> and walking shadow page cache for 1024 times, it is terrible
>>>>
>>>> In fact, if it is the repeat string instructions emulated and it is not a
>>>> IO/MMIO access, we can zap all the corresponding shadow pages and return to the
>>>> guest, then the mapping can became writable and directly write the page
>>>>
>>
>>
>>> So this patch does two independent things as far as I can see. First it
>>> stops reentering guest if rep instruction is done on memory and second
>>
>> No.
>> Oppositely, it enters guest as soon as possible if rep instruction is done
>> on memory ;-)
> Oops. Indeed. I read it other way around. So why not just return
> X86EMUL_UNHANDLEABLE from emulator_write_emulated_onepage() which should
> have the same effect?
>

It seams not, the count register(RCX) is not decreased, and redundant work
need to be done by handling EMULATION_FAILED.
So, emulator_write_emulated_onepage() is not a good place i think. :-)