Re: [PATCH 5/5] KVM: MMU: fast invalid all mmio sptes

Gleb Natapov
2013-03-18T13:27:57Z

On Mon, Mar 18, 2013 at 09:25:10PM +0800, Xiao Guangrong wrote:
> On 03/18/2013 09:19 PM, Gleb Natapov wrote:
> > On Mon, Mar 18, 2013 at 09:09:43PM +0800, Xiao Guangrong wrote:
> >> On 03/18/2013 08:46 PM, Gleb Natapov wrote:
> >>> On Mon, Mar 18, 2013 at 08:29:29PM +0800, Xiao Guangrong wrote:
> >>>> On 03/18/2013 05:13 PM, Gleb Natapov wrote:
> >>>>> On Mon, Mar 18, 2013 at 04:08:50PM +0800, Xiao Guangrong wrote:
> >>>>>> On 03/17/2013 11:02 PM, Gleb Natapov wrote:
> >>>>>>> On Fri, Mar 15, 2013 at 11:29:53PM +0800, Xiao Guangrong wrote:
> >>>>>>>> This patch tries to introduce a very simple and scale way to invalid all
> >>>>>>>> mmio sptes - it need not walk any shadow pages and hold mmu-lock
> >>>>>>>>
> >>>>>>>> KVM maintains a global mmio invalid generation-number which is stored in
> >>>>>>>> kvm->arch.mmio_invalid_gen and every mmio spte stores the current global
> >>>>>>>> generation-number into his available bits when it is created
> >>>>>>>>
> >>>>>>>> When KVM need zap all mmio sptes, it just simply increase the global
> >>>>>>>> generation-number. When guests do mmio access, KVM intercepts a MMIO #PF
> >>>>>>>> then it walks the shadow page table and get the mmio spte. If the
> >>>>>>>> generation-number on the spte does not equal the global generation-number,
> >>>>>>>> it will go to the normal #PF handler to update the mmio spte
> >>>>>>>>
> >>>>>>>> Since 19 bits are used to store generation-number on mmio spte, the
> >>>>>>>> generation-number can be round after 33554432 times. It is large enough
> >>>>>>>> for nearly all most cases, but making the code be more strong, we zap all
> >>>>>>>> shadow pages when the number is round
> >>>>>>>>
> >>>>>>> Very nice idea, but why drop Takuya patches instead of using
> >>>>>>> kvm_mmu_zap_mmio_sptes() when generation number overflows.
> >>>>>>
> >>>>>> I am not sure whether it is still needed. Requesting to zap all mmio sptes for
> >>>>>> more than 500000 times is really really rare, it nearly does not happen.
> >>>>>> (By the way, 33554432 is wrong in the changelog, i just copy that for my origin
> >>>>>> implantation.) And, after my patch optimizing zapping all shadow pages,
> >>>>>> zap-all-sps should not be a problem anymore since it does not take too much lock
> >>>>>> time.
> >>>>>>
> >>>>>> Your idea?
> >>>>>>
> >>>>> I expect 500000 to become less since I already had plans to store some
> >>>>
> >>>> Interesting, just curious, what are the plans? ;)
> >>>>
> >>> Currently we uses pio to signal that work is pending to virtio devices. The
> >>> requirement is that signaling should be fast and PIO is fast since there
> >>> is not need to emulate instruction. PCIE though is not really designed
> >>> with PIO in mind, so we will have to use MMIO to do signaling. To avoid
> >>> instruction emulation I thought about making guest access these devices using
> >>> predefined variety of MOV instruction so that emulation can be skipped.
> >>> The idea is to mark mmio spte to know that emulation is not needed.
> >>
> >> How to know page-fault is caused by the predefined instruction?
> >>
> > Only predefined phys address rages will be accessed that way. If page
> > fault is in a range we assume the knows instruction is used.
>
> That means the access can be identified by the gfn, why need cache
> other things into mmio spte?
Two not search through memory ranges on each access.

--
Gleb.