linux-kernel - Re: [PATCH v3 0/6] KVM: MMU: fast invalidate all mmio sptes

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Date:	Wed, 19 Jun 2013 13:08:10 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>,
	Xiao Guangrong <xiaoguangrong.eric@...il.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	avi.kivity@...il.com, mtosatti@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 0/6] KVM: MMU: fast invalidate all mmio sptes

Il 10/06/2013 19:03, Gleb Natapov ha scritto:
> On Mon, Jun 10, 2013 at 10:43:52PM +0900, Takuya Yoshikawa wrote:
>> On Mon, 10 Jun 2013 16:39:37 +0800
>> Xiao Guangrong <xiaoguangrong.eric@...il.com> wrote:
>>
>>> On 06/10/2013 03:56 PM, Gleb Natapov wrote:
>>>> On Fri, Jun 07, 2013 at 04:51:22PM +0800, Xiao Guangrong wrote:
>>
>>>> Looks good to me, but doesn't tis obsolete kvm_mmu_zap_mmio_sptes() and
>>>> sp->mmio_cached, so they should be removed as part of the patch series?
>>>
>>> Yes, i agree, they should be removed. :)
>>
>> I'm fine with removing it but please make it clear that you all agree
>> on the same basis.
>>
>> Last time, Paolo mentioned the possibility to use some bits of spte for
>> other things.  The suggestion there was to keep sp->mmio_cached code
>> for the time we would need to reduce the bits for generation numbers.
>>
>> Do you think that zap_all() is now preemptible and can treat the
>> situation reasonably well as the current kvm_mmu_zap_mmio_sptes()?
>>
>> One downside is the need to zap unrelated shadow pages, but if this case
>> is really very rare, yes I agree, it should not be a problem: it depends
>> on how many bits we can use.
>>
>> Just please reconfirm.
>>
> That was me who mention the possibility to use some bits of spte for
> other things. But for now I have a use for one bit only. Now that you
> have reminded me about that discussion I am not so sure we want to
> remove kvm_mmu_zap_mmio_sptes(), but on the other hand it is non
> preemptable, so large number of mmio sptes can cause soft lockups.
> zap_all() is better in this regards now.

I asked Gleb on IRC, and he's fine with applying patch 7 too (otherwise
there's hardly any benefit, because kvm_mmu_zap_mmio_sptes is
non-preemptable).

I'm also changing the -13 to -150 since it's quite easy to generate 150
calls to KVM_SET_USER_MEMORY_REGION.  Using QEMU, and for a pretty basic
guest with virtio-net, IDE controller and VGA you get:

- 9-10 calls before starting the guest, depending on the guest memory size

- around 25 during the BIOS

- around 20 during kernel boot

- 34 during a single dump of the 64 KB ROM from a virtio-net device.

Paolo
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