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

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Message-ID: <483eb20c-7302-4733-a15f-21d140396919@zytor.com>
Date: Thu, 24 Apr 2025 18:15:36 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Jürgen Groß <jgross@...e.com>,
        Xin Li
 <xin@...or.com>, linux-kernel@...r.kernel.org,
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Subject: Re: [RFC PATCH v2 21/34] x86/msr: Utilize the alternatives mechanism
 to write MSR

On 4/24/25 01:14, Jürgen Groß wrote:
>>
>> Actually, that is how we get this patch with the existing alternatives
>> infrastructure.  And we took a step further to also remove the pv_ops
>> MSR APIs...
> 
> And this is what I'm questioning. IMHO this approach is adding more
> code by removing the pv_ops MSR_APIs just because "pv_ops is bad". And
> I believe most refusal of pv_ops is based on no longer valid reasoning.
> 

pvops are a headache because it is effectively a secondary alternatives 
infrastructure that is incompatible with the alternatives one...

>> It looks to me that you want to add a new facility to the alternatives
>> infrastructure first?
> 
> Why would we need a new facility in the alternatives infrastructure?

I'm not sure what Xin means with "facility", but a key motivation for 
this is to:

a. Avoid using the pvops for MSRs when on the only remaining user 
thereof (Xen) is only using it for a very small subset of MSRs and for 
the rest it is just overhead, even for Xen;

b. Being able to do wrmsrns immediate/wrmsrns/wrmsr and rdmsr 
immediate/rdmsr alternatives.

Of these, (b) is by far the biggest motivation. The architectural 
direction for supervisor states is to avoid ad hoc and XSAVES ISA and 
instead use MSRs. The immediate forms are expected to be significantly 
faster, because they make the MSR index available at the very beginning 
of the pipeline instead of at a relatively late stage.

	-hpa