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

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Message-ID: <5147177B.1090609@linux.vnet.ibm.com>
Date:	Mon, 18 Mar 2013 21:32:43 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 5/5] KVM: MMU: fast invalid all mmio sptes

On 03/18/2013 09:27 PM, Gleb Natapov wrote:
> 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.

Aha... got it. Thank you! ;)

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