On 07/27/2011 03:51 PM, Gleb Natapov wrote: > On Wed, Jul 27, 2011 at 02:32:43PM +0800, Xiao Guangrong wrote: >> 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. > The only difference is that with your approach one rep is emulated and then > control goes back to a guest. With EMULATION_FAILED kvm returns to a guest > immediately, so RCX shouldn't be decreased. There shouldn't a be big difference > performance wise and if there is it is likely on EMULATION_FAILED side. > Last but not least emulate.c knows nothing about the hack. > Umm. it is reasonable, i'll update this patch. Thanks, Gleb! -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
