Re: [PATCH RFC 4/7] kvm: x86: Add new ioctls for XSAVE extension

[Sean Christopherson <seanjc@google.com>]

On Sun, Feb 07, 2021, Jing Liu wrote:
> The static xstate buffer kvm_xsave contains the extended register
> states, but it is not enough for dynamic features with large state.
> 
> Introduce a new capability called KVM_CAP_X86_XSAVE_EXTENSION to
> detect if hardware has XSAVE extension (XFD). Meanwhile, add two
> new ioctl interfaces to get/set the whole xstate using struct
> kvm_xsave_extension buffer containing both static and dynamic
> xfeatures. Reuse fill_xsave and load_xsave for both cases.
> 
> Signed-off-by: Jing Liu <jing2.liu@linux.intel.com>
> ---
>  arch/x86/include/uapi/asm/kvm.h |  5 +++
>  arch/x86/kvm/x86.c              | 70 +++++++++++++++++++++++++--------
>  include/uapi/linux/kvm.h        |  8 ++++
>  3 files changed, 66 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> index 89e5f3d1bba8..bf785e89a728 100644
> --- a/arch/x86/include/uapi/asm/kvm.h
> +++ b/arch/x86/include/uapi/asm/kvm.h
> @@ -362,6 +362,11 @@ struct kvm_xsave {
>  	__u32 region[1024];
>  };
>  
> +/* for KVM_CAP_XSAVE_EXTENSION */
> +struct kvm_xsave_extension {
> +	__u32 region[3072];

Fool me once, shame on you (Intel).  Fool me twice, shame on me (KVM).

As amusing as kvm_xsave_really_extended would be, the required size should be
discoverable, not hardcoded.  Nothing prevents a hardware vendor from inventing
a newfangled feature that requires yet more space.

As an alternative to adding a dedicated capability, can we leverage
GET_SUPPORTED_CPUID, leaf CPUID.0xD, to enumerate the minimum required size and
state that the new ioctl() is available if the min size is greater than 1024?
Or is that unnecessarily convoluted...