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

Jürgen Groß
2025-04-25T07:01:33Z

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On 25.04.25 05:44, H. Peter Anvin wrote:
> On 4/24/25 18:15, H. Peter Anvin wrote:
>> 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 I believe most refusal of pv_ops is based on no longer valid reasoning.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.
>>
>
> Note that to support the immediate forms, we *must* do these inline, or the
> const-ness of the MSR index -- which applies to by far the vast majority of MSR
> references -- gets lost. pvops does exactly that.
>
> Furthermore, the MSR immediate instructions take a 64-bit number in a single
> register; as these instructions are by necessity relatively long, it makes sense
> for the alternative sequence to accept a 64-bit input register and do the %eax/
> %edx shuffle in the legacy fallback code... we did a bunch of experiments to see
> what made most sense.

Yes, I understand that.

And I'm totally in favor of Xin's rework of the MSR low level functions.

Inlining the MSR access instructions with pv_ops should not be very
complicated. We do that with other instructions (STI/CLI, PTE accesses)
today, so this is no new kind of functionality.

I could have a try writing a patch achieving that, but I would only start
that work in case you might consider taking it instead of Xin's patch
removing the pv_ops usage for rdmsr/wrmsr. In case it turns out that my
version results in more code changes than Xin's patch, I'd be fine to drop
my patch, of course.

Juergen

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