linux-kernel - Re: [RFC PATCH v2 02/11] KVM: x86: Advertise CPUID.7.2.EDX and RRSBA

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Date:   Tue, 16 May 2023 15:03:15 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Chao Gao <chao.gao@...el.com>
Cc:     kvm@...r.kernel.org, Jiaan Lu <jiaan.lu@...el.com>,
        Zhang Chen <chen.zhang@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 02/11] KVM: x86: Advertise CPUID.7.2.EDX and
 RRSBA_CTRL support

On 5/16/2023 11:01 AM, Chao Gao wrote:
> On Tue, May 16, 2023 at 10:22:22AM +0800, Xiaoyao Li wrote:
>>>> I think we need to fix this bug at first.
>>>
>>> I have no idea how to fix the "bug" without intercepting the MSR. The
>>> performance penalty makes me think intercepting the MSR is not a viable
>>> solution.
>>
>> I thought correctness always takes higher priority over performance.
> 
> It is generally true. however, there are situations where we should make
> trade-offs between correctness and other factors (like performance):
> 
> E.g., instructions without control bits, to be 100% compliant with CPU
> spec, in theory, VMMs can trap/decode every instruction and inject #UD
> if a guest tries to use some instructions it shouldn't.

This is the virtualization hole. IMHO, they are different things.

Pass through MSR_IA32_SPEC_CTRL was introduced in commit d28b387fb74d 
("KVM/VMX: Allow direct access to MSR_IA32_SPEC_CTRL"). At that time 
there was only a few bits defined, and the changelog called out that

   No attempt is made to handle STIBP here, intentionally. Filtering
   STIBP may be added in a future patch, which may require trapping all
   writes if we don't want to pass it through directly to the guest.

Per my undesrstanding, it implied that we need to re-visit it when more 
bits added instead of following the pass-through design siliently.