* CPUID.40000001H:EDX.KVM_HINTS_REALTIME[bit 0]
@ 2020-10-20 17:12 Jim Mattson
2020-10-21 7:03 ` Wanpeng Li
0 siblings, 1 reply; 5+ messages in thread
From: Jim Mattson @ 2020-10-20 17:12 UTC (permalink / raw)
To: kvm list
Per the KVM_GET_SUPPORTED_CPUID ioctl, the KVM_HINTS_REALTIME CPUID
bit is never supported.
Is this just an oversight?
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: CPUID.40000001H:EDX.KVM_HINTS_REALTIME[bit 0]
2020-10-20 17:12 CPUID.40000001H:EDX.KVM_HINTS_REALTIME[bit 0] Jim Mattson
@ 2020-10-21 7:03 ` Wanpeng Li
2020-10-21 14:57 ` Vitaly Kuznetsov
0 siblings, 1 reply; 5+ messages in thread
From: Wanpeng Li @ 2020-10-21 7:03 UTC (permalink / raw)
To: Jim Mattson; +Cc: kvm list
On Wed, 21 Oct 2020 at 14:47, Jim Mattson <jmattson@google.com> wrote:
>
> Per the KVM_GET_SUPPORTED_CPUID ioctl, the KVM_HINTS_REALTIME CPUID
> bit is never supported.
>
> Is this just an oversight?
It is a performance hint, not a PV feature and doesn't depend on KVM.
Wanpeng
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: CPUID.40000001H:EDX.KVM_HINTS_REALTIME[bit 0]
2020-10-21 7:03 ` Wanpeng Li
@ 2020-10-21 14:57 ` Vitaly Kuznetsov
2020-10-21 17:37 ` Jim Mattson
0 siblings, 1 reply; 5+ messages in thread
From: Vitaly Kuznetsov @ 2020-10-21 14:57 UTC (permalink / raw)
To: Wanpeng Li; +Cc: kvm list, Jim Mattson
Wanpeng Li <kernellwp@gmail.com> writes:
> On Wed, 21 Oct 2020 at 14:47, Jim Mattson <jmattson@google.com> wrote:
>>
>> Per the KVM_GET_SUPPORTED_CPUID ioctl, the KVM_HINTS_REALTIME CPUID
>> bit is never supported.
>>
>> Is this just an oversight?
>
> It is a performance hint, not a PV feature and doesn't depend on KVM.
>
True, but personally I'd prefer it to be reported in
KVM_GET_SUPPORTED_CPUID too, that would mean that userspace is in its
right to set it, just like any other PV 'thingy', even if just for
consistency.
--
Vitaly
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: CPUID.40000001H:EDX.KVM_HINTS_REALTIME[bit 0]
2020-10-21 14:57 ` Vitaly Kuznetsov
@ 2020-10-21 17:37 ` Jim Mattson
2020-10-22 3:14 ` Wanpeng Li
0 siblings, 1 reply; 5+ messages in thread
From: Jim Mattson @ 2020-10-21 17:37 UTC (permalink / raw)
To: Vitaly Kuznetsov; +Cc: Wanpeng Li, kvm list
On Wed, Oct 21, 2020 at 7:57 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> Wanpeng Li <kernellwp@gmail.com> writes:
>
> > On Wed, 21 Oct 2020 at 14:47, Jim Mattson <jmattson@google.com> wrote:
> >>
> >> Per the KVM_GET_SUPPORTED_CPUID ioctl, the KVM_HINTS_REALTIME CPUID
> >> bit is never supported.
> >>
> >> Is this just an oversight?
> >
> > It is a performance hint, not a PV feature and doesn't depend on KVM.
> >
>
> True, but personally I'd prefer it to be reported in
> KVM_GET_SUPPORTED_CPUID too, that would mean that userspace is in its
> right to set it, just like any other PV 'thingy', even if just for
> consistency.
I agree. The documentation says:
This ioctl returns x86 cpuid features which are supported by both the
hardware and kvm in its default configuration.
If the bit isn't set, then it is not supported, and a well-behaved
userspace will not set it.
Now, if the bit were in a completely different CPUID leaf (not a
KVM-defined leaf), then one could argue that it falls outside of the
realm of KVM_GET_SUPPORTED_CPUID. But it isn't, is it?
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: CPUID.40000001H:EDX.KVM_HINTS_REALTIME[bit 0]
2020-10-21 17:37 ` Jim Mattson
@ 2020-10-22 3:14 ` Wanpeng Li
0 siblings, 0 replies; 5+ messages in thread
From: Wanpeng Li @ 2020-10-22 3:14 UTC (permalink / raw)
To: Jim Mattson; +Cc: Vitaly Kuznetsov, kvm list
On Thu, 22 Oct 2020 at 01:37, Jim Mattson <jmattson@google.com> wrote:
>
> On Wed, Oct 21, 2020 at 7:57 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> >
> > Wanpeng Li <kernellwp@gmail.com> writes:
> >
> > > On Wed, 21 Oct 2020 at 14:47, Jim Mattson <jmattson@google.com> wrote:
> > >>
> > >> Per the KVM_GET_SUPPORTED_CPUID ioctl, the KVM_HINTS_REALTIME CPUID
> > >> bit is never supported.
> > >>
> > >> Is this just an oversight?
> > >
> > > It is a performance hint, not a PV feature and doesn't depend on KVM.
> > >
> >
> > True, but personally I'd prefer it to be reported in
> > KVM_GET_SUPPORTED_CPUID too, that would mean that userspace is in its
> > right to set it, just like any other PV 'thingy', even if just for
> > consistency.
>
> I agree. The documentation says:
>
> This ioctl returns x86 cpuid features which are supported by both the
> hardware and kvm in its default configuration.
>
> If the bit isn't set, then it is not supported, and a well-behaved
> userspace will not set it.
>
> Now, if the bit were in a completely different CPUID leaf (not a
> KVM-defined leaf), then one could argue that it falls outside of the
> realm of KVM_GET_SUPPORTED_CPUID. But it isn't, is it?
Fair enough, just sent out a patch.
Wanpeng
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2020-10-22 3:14 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
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2020-10-20 17:12 CPUID.40000001H:EDX.KVM_HINTS_REALTIME[bit 0] Jim Mattson
2020-10-21 7:03 ` Wanpeng Li
2020-10-21 14:57 ` Vitaly Kuznetsov
2020-10-21 17:37 ` Jim Mattson
2020-10-22 3:14 ` Wanpeng Li
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