linux-kernel - Re: [PATCH] KVM: X86: Expose KVM_HINTS_REALTIME in KVM_GET_SUPPORTED

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Message-ID: <CALMp9eQyJXko_CKPgg4xRDCsvOmA8zJvrg_kmU6weu=MwKBv0w@mail.gmail.com>
Date:   Thu, 22 Oct 2020 10:13:40 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: X86: Expose KVM_HINTS_REALTIME in KVM_GET_SUPPORTED_CPUID

On Thu, Oct 22, 2020 at 9:37 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 22/10/20 18:35, Jim Mattson wrote:
> > On Thu, Oct 22, 2020 at 6:02 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
> >>
> >> On 22/10/20 03:34, Wanpeng Li wrote:
> >>> From: Wanpeng Li <wanpengli@...cent.com>
> >>>
> >>> 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@...gle.com>
> >>> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> >>
> >> 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?