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

Xiao Guangrong
2013-03-18T13:25:22Z

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?