On 05/22/2013 04:54 PM, Gleb Natapov wrote: > On Wed, May 22, 2013 at 04:46:04PM +0800, Xiao Guangrong wrote: >> On 05/22/2013 02:34 PM, Gleb Natapov wrote: >>> On Tue, May 21, 2013 at 10:33:30PM -0300, Marcelo Tosatti wrote: >>>> On Tue, May 21, 2013 at 11:39:03AM +0300, Gleb Natapov wrote: >>>>>> Any pages with stale information will be zapped by kvm_mmu_zap_all(). >>>>>> When that happens, page faults will take place which will automatically >>>>>> use the new generation number. >>>>>> >>>>>> So still not clear why is this necessary. >>>>>> >>>>> This is not, strictly speaking, necessary, but it is the sane thing to do. >>>>> You cannot update page's generation number to prevent it from been >>>>> destroyed since after kvm_mmu_zap_all() completes stale ptes in the >>>>> shadow page may point to now deleted memslot. So why build shadow page >>>>> table with a page that is in a process of been destroyed? >>>> >>>> OK, can this be introduced separately, in a later patch, with separate >>>> justification, then? >>>> >>>> Xiao please have the first patches of the patchset focus on the problem >>>> at hand: fix long mmu_lock hold times. >>>> >>>>> Not sure what you mean again. We flush TLB once before entering this function. >>>>> kvm_reload_remote_mmus() does this for us, no? >>>> >>>> kvm_reload_remote_mmus() is used as an optimization, its separate from the >>>> problem solution. >>>> >>>>>> >>>>>> What was suggested was... go to phrase which starts with "The only purpose >>>>>> of the generation number should be to". >>>>>> >>>>>> The comment quoted here does not match that description. >>>>>> >>>>> The comment describes what code does and in this it is correct. >>>>> >>>>> You propose to not reload roots right away and do it only when root sp >>>>> is encountered, right? So my question is what's the point? There are, >>>>> obviously, root sps with invalid generation number at this point, so >>>>> reload will happen regardless in kvm_mmu_prepare_zap_page(). So why not >>>>> do it here right away and avoid it in kvm_mmu_prepare_zap_page() for >>>>> invalid and obsolete sps as I proposed in one of my email? >>>> >>>> Sure. But Xiao please introduce that TLB collapsing optimization as a >>>> later patch, so we can reason about it in a more organized fashion. >>> >>> So, if I understand correctly, you are asking to move is_obsolete_sp() >>> check from kvm_mmu_get_page() and kvm_reload_remote_mmus() from >>> kvm_mmu_invalidate_all_pages() to a separate patch. Fine by me, but if >>> we drop kvm_reload_remote_mmus() from kvm_mmu_invalidate_all_pages() the >>> call to kvm_mmu_invalidate_all_pages() in emulator_fix_hypercall() will >>> become nop. But I question the need to zap all shadow pages tables there >>> in the first place, why kvm_flush_remote_tlbs() is not enough? >> >> I do not know too... I even do no know why kvm_flush_remote_tlbs >> is needed. :( > We changed the content of an executable page, we need to flush instruction > cache of all vcpus to not use stale data, so my suggestion to call I thought the reason is about icache too but icache is automatically flushed on x86, we only need to invalidate the prefetched instructions by executing a serializing operation. See the SDM in the chapter of "8.1.3 Handling Self- and Cross-Modifying Code" > kvm_flush_remote_tlbs() is obviously incorrect since this flushes tlb, > not instruction cache, but why kvm_reload_remote_mmus() would flush > instruction cache? kvm_reload_remote_mmus do not have any help i think. I find that this change is introduced by commit: 7aa81cc0 and I have added Anthony in the CC. I also find some discussions related to calling kvm_reload_remote_mmus(): > > But if the instruction is architecture dependent, and you run on the > wrong architecture, now you have to patch many locations at fault time, > introducing some nasty runtime code / data cache overlap performance > problems. Granted, they go away eventually. > We're addressing that by blowing away the shadow cache and holding the big kvm lock to ensure SMP safety. Not a great thing to do from a performance perspective but the whole point of patching is that the cost is amortized. (http://kerneltrap.org/mailarchive/linux-kernel/2007/9/14/260288) But i can not understand... -- 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
