Re: [PATCH] x86/cpufeatures: Force disable X86_FEATURE_ENQCMD and remove update_pasid()

[Andy Lutomirski]

On 6/2/21 1:37 PM, Luck, Tony wrote:
>>> ... so on a PASID system, your trivial reproducer would theoretically
>>> fire the same way and corrupt FPU state just as well.
>>
>> This is worse and you can't selftest it because the IPI can just hit in
>> the middle of _any_ FPU state operation and corrupt state.
> 
> That sounds like we should abandon the "IPI all the other threads
> to force enable the PASID for them" approach. It would just be a
> nightmare of papering over cracks when the IPI was delivered at
> some inconvenient moment when the recipient was in the middle
> of touching xsave state.
> 
> I've told Fenghua to dig out the previous iteration of this patch where
> the plan was to lazily fix the PASID_MSR in other threads in the #GP
> handler.

Blech.  Also this won't work for other PASID-like features.

I have a half-written patch to fix this up for real.  Stay tuned.

> Seems like a better direction than trying to fix the IPI method. The virtualization
> folks will like this way more because IPI in guest causes a couple of VMEXIT
> so is somewhat expensive.

It happens at most once per PASID-using process.