Re: [RFC PATCH v2 06/34] x86/msr: Use the alternatives mechanism to read PMC

Xin Li
2025-04-22T09:13:54Z

On 4/22/2025 1:38 AM, Jürgen Groß wrote:
> On 22.04.25 10:21, Xin Li (Intel) wrote:
>> To eliminate the indirect call overhead introduced by the pv_ops API,
>> use the alternatives mechanism to read PMC:
>
> Which indirect call overhead? The indirect call is patched via the
> alternative mechanism to a direct one.
>

See below.

>>
>>      1) When built with !CONFIG_XEN_PV, X86_FEATURE_XENPV becomes a
>>         disabled feature, preventing the Xen PMC read code from being
>>         built and ensuring the native code is executed unconditionally.
>
> Without CONFIG_XEN_PV CONFIG_PARAVIRT_XXL is not selected, resulting in
> native code anyway.

Yes, this is kept in this patch, but in a little different way.

>
>>
>>      2) When built with CONFIG_XEN_PV:
>>
>>         2.1) If not running on the Xen hypervisor (!X86_FEATURE_XENPV),
>>              the kernel runtime binary is patched to unconditionally
>>              jump to the native PMC read code.
>>
>>         2.2) If running on the Xen hypervisor (X86_FEATURE_XENPV), the
>>              kernel runtime binary is patched to unconditionally jump
>>              to the Xen PMC read code.
>>
>> Consequently, remove the pv_ops PMC read API.
>
> I don't see the value of this patch.
>
> It adds more #ifdef and code lines without any real gain.
>
> In case the x86 maintainers think it is still worth it, I won't object.

I think we want to totally bypass pv_ops in the case 2.1).

Do you mean the indirect call is patched to call native code *directly*
for 2.1? I don't know it, can you please elaborate?

AFAIK, Xen PV has been the sole user of pv_ops for nearly 20 years. This
raises significant doubts about whether pv_ops provides Linux with the
value of being a well-abstracted "CPU" or "Platform". And the x86
maintainers have said that it's a maintenance nightmare.

Thanks!
Xin