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

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Message-ID: <871rhq4ckl.fsf@vitty.brq.redhat.com>
Date:   Thu, 22 Oct 2020 15:25:46 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: X86: Expose KVM_HINTS_REALTIME in KVM_GET_SUPPORTED_CPUID

Paolo Bonzini <pbonzini@...hat.com> writes:

> 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.
>
> (But maybe this was discussed already; if so, please point me to the
> previous discussion).
>

Not for KVM_GET_SUPPORTED_CPUID but for KVM_GET_SUPPORTED_HV_CPUID: just
copying all the bits blindly is incorrect as e.g. SYNIC needs to be
enabled with KVM_CAP_HYPERV_SYNIC[2]. KVM PV features also have
pre-requisites, e.g. KVM_ASYNC_PF_DELIVERY_AS_INT requires an irqchip so
again copy/paste may not work.

-- 
Vitaly