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. > So, emulator_write_emulated_onepage() is not a good place i think. :-) -- 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
