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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ZoyAkkZjnGmwlVCS@google.com>
Date: Mon, 8 Jul 2024 17:13:06 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Maxim Levitsky <mlevitsk@...hat.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 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.