linux-kernel - Re: [PATCH 25/25] KVM: x86: Add CPUID bits missing from KVM_GET_SUPPORTED

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Message-ID: <33f21483-0e55-4370-8405-12ed7439c3e9@intel.com>
Date: Fri, 13 Sep 2024 11:57:00 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>, kvm@...r.kernel.org,
 kai.huang@...el.com, isaku.yamahata@...il.com,
 tony.lindgren@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 25/25] KVM: x86: Add CPUID bits missing from
 KVM_GET_SUPPORTED_CPUID

On 9/13/2024 12:42 AM, Sean Christopherson wrote:
> On Thu, Sep 12, 2024, Paolo Bonzini wrote:
>> On Thu, Sep 12, 2024 at 4:45 PM Xiaoyao Li <xiaoyao.li@...el.com> wrote:
>>>> KVM is not going to have any checks, it's only going to pass the
>>>> CPUID to the TDX module and return an error if the check fails
>>>> in the TDX module.
>>>
>>> If so, new feature can be enabled for TDs out of KVM's control.
>>>
>>> Is it acceptable?
>>
>> It's the same as for non-TDX VMs, I think it's acceptable.
> 
> No?  IIUC, it's not the same.
> 
> E.g. KVM doesn't yet support CET, and while userspace can enumerate CET support
> to VMs all it wants, guests will never be able to set CR4.CET and thus can't
> actually enable CET.
> 
> IIUC, the proposal here is to allow userspace to configure the features that are
> exposed _and enabled_ for a TDX VM without any enforcement from KVM.
> 
> CET might be a bad example because it looks like it's controlled by TDCS.XFAM, but
> presumably there are other CPUID-based features that would actively enable some
> feature for a TDX VM.
> 
> For HYPERVISOR and TSC_DEADLINE_TIMER, I would much prefer to fix those KVM warts,
> and have already posted patches[1][2] to do exactly that.
> 
> With those out of the way, are there any other CPUID-based features that KVM
> supports, but doesn't advertise?  Ignore MWAIT, it's a special case and isn't
> allowed in TDX VMs anyways.

Actually MWAIT becoems allowed by TDX and it's configurable.

> [1] https://lore.kernel.org/all/20240517173926.965351-34-seanjc@google.com
> [2] https://lore.kernel.org/all/20240517173926.965351-35-seanjc@google.com
>