Re: [PATCH v2 18/23] kvm: x86: Get/set expanded xstate buffer

[Paolo Bonzini]

On 12/21/21 03:45, Wang, Wei W wrote:
> > KVM_GET_XSAVE2 and KVM_SET_XSAVE respectively write and read as many
> > bytes as are returned by KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2), when
> > invoked on the vm file descriptor.  Currently,
> > KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will only return a value that is
> > greater than 4096 bytes if any dynamic features have been enabled with
> > ``arch_prctl()``; this however may change in the future.
> Would this make people think that KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) doesn’t
> return the value (i.e. return 0) if it is smaller than 4096?
> (i.e. KVM_GET_XSAVE2 doesn't work with size < 4096, which isn’t true)
>
> I plan to just reword a bit:
> Currently, KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will only return a size value,
> and the value is greater than 4096 bytes if any dynamic features have been enabled with
> ``arch_prctl()``. More types of values could be returned in the future.

Next refinement:

The size value returned by KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will 
always be at least 4096.  Currently, it is only greater than 4096 if a 
dynamic feature has been enabled with ``arch_prctl()``, but this may 
change in the future.

(I'm not sure if the first sentence is true in the code, but if not it 
is a bug that has to be fixed :)).

Paolo