linux-kernel - Re: [RFC PATCH v1 10/15] KVM: VMX: Use WRMSRNS or its immediate form when available

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Message-ID: <EAB44BB2-99BB-4D4A-8306-0235D2931E72@zytor.com>
Date: Sat, 12 Apr 2025 16:10:06 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Xin Li <xin@...or.com>, Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
CC: linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linux-hyperv@...r.kernel.org, virtualization@...ts.linux.dev,
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Subject: Re: [RFC PATCH v1 10/15] KVM: VMX: Use WRMSRNS or its immediate form when available

On April 11, 2025 9:32:32 PM PDT, Xin Li <xin@...or.com> wrote:
>On 4/11/2025 2:12 PM, Jim Mattson wrote:
>> Surely, any CPU that has WRMSRNS also supports "Virtualize
>> IA32_SPEC_CTRL," right? Shouldn't we be using that feature rather than
>> swapping host and guest values with some form of WRMSR?
>
>Good question, the simple answer is that they are irrelevant.

Also, *in this specific case* IA32_SPEC_CTRL is architecturally nonserializing, i.e. WRMSR executes as WRMSRNS anyway.