Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <1603330475-7063-1-git-send-email-wanpengli@tencent.com>
 <cfd9d16f-6ddf-60d5-f73d-bb49ccd4055f@redhat.com> <CALMp9eR3Ng-WBrumXaJAecLWZECf-1NfzW+eTA0VxWuAcKAjAA@mail.gmail.com>
 <281bca2d-d534-1032-eed3-7ee7705cb12c@redhat.com>
In-Reply-To: <281bca2d-d534-1032-eed3-7ee7705cb12c@redhat.com>
From:   Jim Mattson <jmattson@google.com>
Date:   Thu, 22 Oct 2020 10:13:40 -0700
Message-ID: <CALMp9eQyJXko_CKPgg4xRDCsvOmA8zJvrg_kmU6weu=MwKBv0w@mail.gmail.com>
Subject: Re: [PATCH] KVM: X86: Expose KVM_HINTS_REALTIME in KVM_GET_SUPPORTED_CPUID
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Wanpeng Li <kernellwp@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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