Re: [PATCH v2 1/6] KVM: x86: Clear all supported MPX xfeatures if they are not all set

["Chang S. Bae" <chang.seok.bae@xxxxxxxxx>]

On 1/12/2023 11:17 AM, Mingwei Zhang wrote:
> But the permitted_xcr0 and supported_xcr0 seems never used
> directly as the parameter of XSETBV, but only for size calculation.

Yeah, I saw that too, and tried to improve it [1]. Maybe this is not a 
big deal in KVM.

> One more question: I am very confused by the implementation of
> xstate_get_guest_group_perm(). It seems fetching the xfeatures from the
> host process (&current->group_leader->thread.fpu). Is this intentional?
> Does that mean in order to enable AMX in the guest we have to enable it
> on the host process first?

Yes, it was designed that QEMU requests permissions via arch_prctl() 
before creating vCPU threads [2][3]. Granted, this feature capability 
will be advertised to the guest. Then, it will *enable* the feature, right?

Thanks,
Chang

[1] 
https://lore.kernel.org/kvm/20220823231402.7839-2-chang.seok.bae@xxxxxxxxx/
[2] https://lore.kernel.org/lkml/87wnmf66m5.ffs@tglx/
[3] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=980fe2fddcff