Re: Thoughts of AMX KVM support based on latest kernel

[Thomas Gleixner]

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?

> we can check the XINUSE state-component bitmap by XGETBV(1). By adding
> more meaning to the existing field: fpstate->in_use, it can be useful
> for KVM to set the XINUSE value.

As I pointed out to Sean, the problem is inconsistent state. Checking
XGETBV(1) cannot make that go away. And I have no idea how you want to
abuse fpstate->in_use for anything related to the XINUSE bitmap. It's a
single bit for a particular purpose and has absolutely nothing to do
with XINUSE. Trying to overload that is just wrong.

If XFD is not trapped then you have exactly three options:

   1) Make it an autosave MSR and grab the guest XFD value from that
      memory to update fpstate and the shadow memory before reenabling
      interrupts

   2) Do the MSR read how I suggested before reenabling interrupts

   3) Conditionally post XSAVES when fpstate->is_guest == true

Anything else wont work.

Thanks,

        tglx