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

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Message-ID: <1add8de0-8c62-444e-a7d6-5681432d48a9@zytor.com>
Date: Mon, 31 Mar 2025 13:55:11 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        "Xin Li (Intel)" <xin@...or.com>
Cc: linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linux-hyperv@...r.kernel.org, virtualization@...ts.linux.dev,
        linux-edac@...r.kernel.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org, linux-ide@...r.kernel.org,
        linux-pm@...r.kernel.org, bpf@...r.kernel.org, llvm@...ts.linux.dev,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, jgross@...e.com,
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        seanjc@...gle.com, luto@...nel.org, boris.ostrovsky@...cle.com,
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Subject: Re: [RFC PATCH v1 10/15] KVM: VMX: Use WRMSRNS or its immediate form
 when available

On 3/31/25 13:41, Andrew Cooper wrote:
>>
>> That is replace the MSR write to disable speculative execution with a
>> non-serialized WRMSR? Doesn't that mean the WRMSRNS is speculative?
> 
> MSR_SPEC_CTRL is explicitly non-serialising, even with a plain WRMSR.
> 
> non-serialising != non-speculative.
> 
> Although WRMSRNS's precise statement on the matter of
> non-speculativeness is woolly, given an intent to optimise it some more
> in the future.
> 

To be specific, "serializing" is a much harder statement than 
"non-speculative."

For architecturally non-serializing MSRs, WRMSRNS and WRMSR are 
equivalent (or to put it differently, WRMSR acts like WRMSRNS.)

The advantage with making them explicitly WRMSRNS is that it allows for 
the substitution of the upcoming immediate forms.

	-hpa