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? > After this patch, we only need to emulate one time for a rep instruction. > > > it drops shadow pages if access to shadowed page table is rep. Why not > > separate those in different patches? > > Umm, i will zap shadow page firstly and stop emulation rep instruction in > the second patch. > > > BTW not entering guest periodically > > increases interrupt latencies. > > Oppositely, It reduces the latencies. :-) > > > Why not zap shadow, make page writable > > and reenter the guest instead of emulation, it should be much faster (do we > > care to optimize for old cpus by complicating the code anyway?). > > > > We do better lazily update the mapping to writable, it can be done by the later access. -- 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
