linux-kernel - Re: [PATCH v2 39/49] KVM: x86: Extract code for generating per-entry emulated CPUID information

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Message-ID: <d8ad4c17b01dd3bd3cdceba6712907ed2c7a08e6.camel@redhat.com>
Date: Wed, 24 Jul 2024 14:00:26 -0400
From: Maxim Levitsky <mlevitsk@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Vitaly Kuznetsov
 <vkuznets@...hat.com>,  kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Hou Wenlong <houwenlong.hwl@...group.com>, Kechen Lu <kechenl@...dia.com>,
 Oliver Upton <oliver.upton@...ux.dev>, Binbin Wu
 <binbin.wu@...ux.intel.com>, Yang Weijiang <weijiang.yang@...el.com>,
 Robert Hoo <robert.hoo.linux@...il.com>
Subject: Re: [PATCH v2 39/49] KVM: x86: Extract code for generating
 per-entry emulated CPUID information

On Mon, 2024-07-08 at 17:13 -0700, Sean Christopherson wrote:
> On Thu, Jul 04, 2024, Maxim Levitsky wrote:
> > On Fri, 2024-05-17 at 10:39 -0700, Sean Christopherson wrote:
> > PS: I spoke with Paolo about the meaning of KVM_GET_EMULATED_CPUID, because
> > it is not clear from the documentation what it does, or what it supposed to
> > do because qemu doesn't use this IOCTL.
> > 
> > So this ioctl is meant to return a static list of CPU features which *can* be
> > emulated by KVM, if the cpu doesn't support them, but there is a cost to it,
> > so they should not be enabled by default.
> > 
> > This means that if you run 'qemu -cpu host', these features (like rdpid) will
> > only be enabled if supported by the host cpu, however if you explicitly ask
> > qemu for such a feature, like 'qemu -cpu host,+rdpid', qemu should not warn
> > if the feature is not supported on host cpu but can be emulated (because kvm
> > can emulate the feature, which is stated by KVM_GET_EMULATED_CPUID ioctl).
> > 
> > Qemu currently doesn't support this but the support can be added.
> > 
> > So I think that the two ioctls should be redefined as such:
> > 
> > KVM_GET_SUPPORTED_CPUID - returns all CPU features that are supported by KVM,
> > supported by host hardware, or that KVM can efficiently emulate.
> > 
> > 
> > KVM_GET_EMULATED_CPUID - returns all CPU features that KVM *can* emulate if
> > the host cpu lacks support, but emulation is not efficient and thus these
> > features should be used with care when not supported by the host (e.g only
> > when the user explicitly asks for them).
> 
> Yep, that aligns with how I view the ioctls (I haven't read the documentaion,
> mainly because I have a terrible habit of never reading docs).
> 
> > I can post a patch to fix this or you can add something like that to your
> > patch series if you prefer.
> 
> Go ahead and post a patch, assuming it's just a documentation update.
> 
OK, will do.

Best regards,
	Maxim Levitsky