Re: [PATCH v2] kvm/svm: PKU not currently supported

[Paolo Bonzini <pbonzini@redhat.com>]

On 19/12/19 21:32, Sean Christopherson wrote:
> On Thu, Dec 19, 2019 at 02:17:59PM -0600, John Allen wrote:
>> Current SVM implementation does not have support for handling PKU. Guests
>> running on a host with future AMD cpus that support the feature will read
>> garbage from the PKRU register and will hit segmentation faults on boot as
>> memory is getting marked as protected that should not be. Ensure that cpuid
>> from SVM does not advertise the feature.
>>
>> Signed-off-by: John Allen <john.allen@amd.com>
>> ---
>> v2:
>>   -Introduce kvm_x86_ops->pku_supported()
> 
> I like the v1 approach better, it's less code to unwind when SVM gains
> support for virtualizaing PKU.
> 
> The existing cases of kvm_x86_ops->*_supported() in __do_cpuid_func() are
> necessary to handle cases where it may not be possible to expose a feature
> even though it's supported in hardware, host and KVM, e.g. VMX's separate
> MSR-based features and PT's software control to hide it from guest.  In
> this case, hiding PKU is purely due to lack of support in KVM.  The SVM
> series to enable PKU can then delete a single line of SVM code instead of
> having to go back in and do surgery on x86 and VMX.
> 

I sort of liked the V1 approach better, in that I liked using
set_supported_cpuid but I didn't like *removing* features from it.

I think all *_supported() should be removed, and the code moved from
__do_cpuid_func() to set_supported_cpuid.

For now, however, this one is consistent with other features so I am
applying it.

Paolo