Re: [PATCH] KVM: X86: Expose KVM_HINTS_REALTIME in KVM_GET_SUPPORTED_CPUID

[Jim Mattson]

On Thu, Oct 22, 2020 at 9:37 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 22/10/20 18:35, Jim Mattson wrote:
> > On Thu, Oct 22, 2020 at 6:02 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> >>
> >> On 22/10/20 03:34, Wanpeng Li wrote:
> >>> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >>>
> >>> Per KVM_GET_SUPPORTED_CPUID ioctl documentation:
> >>>
> >>> This ioctl returns x86 cpuid features which are supported by both the
> >>> hardware and kvm in its default configuration.
> >>>
> >>> A well-behaved userspace should not set the bit if it is not supported.
> >>>
> >>> Suggested-by: Jim Mattson <jmattson@xxxxxxxxxx>
> >>> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >>
> >> It's common for userspace to copy all supported CPUID bits to
> >> KVM_SET_CPUID2, I don't think this is the right behavior for
> >> KVM_HINTS_REALTIME.
> >
> > That is not how the API is defined, but I'm sure you know that. :-)
>
> Yes, though in my defense :) KVM_HINTS_REALTIME is not a property of the
> kernel, it's a property of the environment that the guest runs in.  This
> was the original reason to separate it from other feature bits in the
> same leaf.
>
> Paolo
>
We don't actually use KVM_GET_SUPPORTED_CPUID at all today. If it's
commonly being misinterpreted as you say, perhaps we should add a
KVM_GET_TRUE_SUPPORTED_CPUID ioctl. Or, perhaps we can just fix this
in the documentation?