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

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Date:   Thu, 22 Oct 2020 18:37:45 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.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 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