Re: [RFC PATCH 3/7] x86: Grant AMX permission for guest

[Paolo Bonzini]

Sorry, hit send on the wrong window.  This is the only patch that will 
require a bit more work.

On 1/18/22 13:52, Paolo Bonzini wrote:
> > @@ -124,6 +150,8 @@ void x86_cpus_init(X86MachineState *x86ms, int 
> > default_cpu_version)
> >       MachineState *ms = MACHINE(x86ms);
> >       MachineClass *mc = MACHINE_GET_CLASS(x86ms);
> > +    /* Request AMX pemission for guest */
> > +    x86_xsave_req_perm();
> >       x86_cpu_set_default_version(default_cpu_version);
>
> This should be done before creating a CPU with support for state 
> component 18.  It happens in kvm_init_vcpu, with the following call stack:
>
>      kvm_init_vcpu
>      kvm_vcpu_thread_fn
>      kvm_start_vcpu_thread
>      qemu_init_vcpu
>      x86_cpu_realizefn
>
> The issue however is that this has to be done before 
> KVM_GET_SUPPORTED_CPUID and KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2).
>
> For the former, you can assume that anything returned by 
> ARCH_GET_XCOMP_GUEST_PERM will be returned by KVM_GET_SUPPORTED_CPUID in 
> CPUID[0xD].EDX:EAX, so you can:
>
> - add it to kvm_arch_get_supported_cpuid

... together with the other special cases (otherwise 
x86_cpu_get_supported_feature_word complains that XTILEDATA is not 
available)

- change kvm_cpu_xsave_init to use host_cpuid instead of 
kvm_arch_get_supported_cpuid.

- call ARCH_REQ_XCOMP_GUEST_PERM from x86_cpu_enable_xsave_components, 
with a conditional like

    if (kvm_enabled()) {
        kvm_request_xsave_components(cpu, mask);
    }

KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) is actually not a problem; the ioctl 
is only called from kvm_arch_init_vcpu and therefore after 
x86_cpu_enable_xsave_components.

Thanks,

Paolo