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 11:01:38 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Xiaoyao Li <xiaoyao.li@...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 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.