Re: Thoughts of AMX KVM support based on latest kernel

[Nakajima, Jun]

> 
> On Nov 16, 2021, at 11:20 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
> Jing,
> 
> On Wed, Nov 10 2021 at 13:01, Liu, Jing2 wrote:
> 
> more thoughts.
> 
>> Once we start passthrough the XFD MSR, we need to save/restore
>> them at VM exit/entry time. If we immediately resume the guest
>> without enabling interrupts/preemptions (exit fast-path), we have no
>> issues. We don't need to save the MSR.
> 
> Correct.
> 
>> The question is how the host XFD MSR is restored while control is in
>> KVM.
>> 
>> The XSAVE(S) instruction saves the (guest) state component[x] as 0 or
>> doesn't save when XFD[x] != 0. Accordingly, XRSTOR(S) cannot restore
>> that (guest state). And it is possible that XFD != 0 and the guest is using
>> extended feature at VM exit;
> 
> You mean on creative guests which just keep AMX state alive and set
> XFD[AMX] = 1 to later restore it to XFD[AMX] = 0?


Typically a (usual) guest saves the AMX state for the previous process and sets XFD[AMX] = 1 for the next at context switch time, and a VM exit can happen anytime, e.g. right after XFD[AMX] = 1. 
But this case is okay because the state is already saved by the guest.

If a (creative) guest wants to set XFD[AMX] = 1 for fun while keeping AMX state alive without saving the AXM state, it may lose the state after VM exit/entry. I think the right thing to do is to avoid such programming in the first place. Let me find out if we can add such notes in the programming references.


--- 
Jun