Re: [PATCH] RFC x86/msr: Use WRMSRNS $imm when available

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 11/08/2025 11:06 am, Jan Beulich wrote:
> On 11.08.2025 11:50, Andrew Cooper wrote:
>> On 11/08/2025 9:16 am, Jan Beulich wrote:
>>> On 09.08.2025 00:20, Andrew Cooper wrote:
>>>> +        "mov %%rax, %%rdx\n\t"
>>>> +        "shr $32, %%rdx\n\t"
>>>> +        ".byte 0x0f,0x01,0xc6", X86_FEATURE_WRMSRNS,
>>>> +
>>>> +        [msr] "i" (msr), "a" (val) : "rcx", "rdx");
>>>         [msr] "i" (msr), "a" (val), "c" (msr) : "rdx");
>>>
>>> allowing the compiler to actually know what's put in %ecx? That'll make
>>> original and 2nd replacement code 10 bytes, better balancing with the 9
>>> bytes of the 1st replacement. And I'd guess that the potentially dead
>>> MOV to %ecx would be hidden in the noise as well.
>> I considered that, but what can the compiler do as a result of knowing %ecx?
> For example ...
>
>> That said, we do need an RDMSR form (which I desperately want to make
>> foo = rdmsr(MSR_BAR) but my cleanup series from 2019 got nowhere), and
>> in a read+write case I suppose the compiler could deduplicate the setup
>> of %ecx.
> ... this. But also simply to use a good pattern (exposing as much as possible
> to the compiler), so there are more good instances of code for future cloning
> from. (In size-optimizing builds, the compiler could further favor ADD/SUB
> over MOV when the two MSRs accessed are relatively close together.)

I have seen the compiler do this in the past, but couldn't reproduce it
for this work.

We specifically do not want any conversion to ADD/SUB, because that
takes our "close to a nop" and makes it no so.

~Andrew