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

[Yang Zhong <yang.zhong@intel.com>]

On Tue, Jan 18, 2022 at 02:06:55PM +0100, Paolo Bonzini wrote:
> 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.
>
  
  Paolo, thanks too much for those detailed steps!
  I have completed the new patch according to those steps, and work well.

  Since this is only big change patch, the next version will be removed RFC.

  Thanks!
  Yang  
  
 
> Thanks,
> 
> Paolo