linux-kernel - Re: [RFC PATCH v2 21/34] x86/msr: Utilize the alternatives mechanism to write MSR

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Message-ID: <D4ADDBA5-D9B8-4DD5-8D83-8CD482700E71@zytor.com>
Date: Fri, 25 Apr 2025 13:12:33 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Jürgen Groß <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>
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Subject: Re: [RFC PATCH v2 21/34] x86/msr: Utilize the alternatives mechanism to write MSR

On April 25, 2025 5:51:27 AM PDT, "Jürgen Groß" <jgross@...e.com> wrote:
>On 25.04.25 14:33, Peter Zijlstra wrote:
>> On Wed, Apr 23, 2025 at 06:05:19PM +0200, Jürgen Groß wrote:
>> 
>>>> It's not a major change, but when it is patched to use the immediate
>>>> form MSR write instruction, it's straightforwardly streamlined.
>>> 
>>> It should be rather easy to switch the current wrmsr/rdmsr paravirt patching
>>> locations to use the rdmsr/wrmsr instructions instead of doing a call to
>>> native_*msr().
>> 
>> Right, just make the Xen functions asm stubs that expect the instruction
>> registers instead of C-abi and ALT_NOT_XEN the thing.
>> 
>> Shouldn't be hard at all.
>
>Correct. And for the new immediate form we can use ALTERNATIVE_3().
>
>
>Juergen

Yes; in the ultimate case there are *four* alternatives, but the concept is the same and again we have it implemented already.