Re: [PATCH v4 0/6] KVM: MMU: performance tweaks for heavy CR0.WP users

[Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>]

On 4/14/2023 10:17 PM, Sean Christopherson wrote:
> On Fri, Apr 14, 2023, Jeremi Piotrowski wrote:
>> On Fri, Apr 14, 2023 at 09:49:28AM -0700, Sean Christopherson wrote:
>>> +Jeremi
>>>
>>
>> Adding myself :)
> 
> /facepalm
> 
> This isn't some mundane detail, Michael!!!
> 
>>> On Fri, Apr 14, 2023, Mathias Krause wrote:
>>
>> ...
>>
>>>> OTOH, the backports give nice speed-ups, ranging from ~2.2 times faster
>>>> for pure EPT (legacy) MMU setups up to 18(!!!) times faster for TDP MMU
>>>> on v5.10.
>>>
>>> Anyone that's enabling the TDP MMU on v5.10 is on their own, we didn't enable the
>>> TDP MMU by default until v5.14 for very good reasons.
>>>
>>>> I backported the whole series down to v5.10 but left out the CR0.WP
>>>> guest owning patch+fix for v5.4 as the code base is too different to get
>>>> all the nuances right, as Sean already hinted. However, even this
>>>> limited backport provides a big performance fix for our use case!
>>>
>>> As a compromise of sorts, I propose that we disable the TDP MMU by default on v5.15,
>>> and backport these fixes to v6.1.  v5.15 and earlier won't get "ludicrous speed", but
>>> I think that's perfectly acceptable since KVM has had the suboptimal behavior
>>> literally since EPT/NPT support was first added.
>>>
>>
>> Disabling TDP MMU for v5.15, and backporting things to v6.1 works for me.
>>
>>> I'm comfortable backporting to v6.1 as that is recent enough, and there weren't
>>> substantial MMU changes between v6.1 and v6.3 in this area.  I.e. I have a decent
>>> level of confidence that we aren't overlooking some subtle dependency.
>>>
>>> For v5.15, I am less confident in the safety of a backport, and more importantly,
>>> I think we should disable the TDP MMU by default to mitigate the underlying flaw
>>> that makes the 18x speedup possible.  That flaw is that KVM can end up freeing and
>>> rebuilding TDP MMU roots every time CR0.WP is toggled or a vCPU transitions to/from
>>> SMM.
>>>
>>
So given that there hasn't been any further comms, I assume we stick to the plan outlined
above: disable tdp_mmu by default for 5.15.

Should that just be a revert of 71ba3f3189c78f756a659568fb473600fd78f207
("KVM: x86: enable TDP MMU by default") or a new patch and more importantly - do you want
to post the patch Sean, or are you busy and would prefer if someone else did?

Jeremi