Re: [PATCH v2 00/12] x86/msr: Inline rdmsr/wrmsr instructions

["H. Peter Anvin" <hpa@zytor.com>]

On 2025-09-30 00:03, Juergen Gross wrote:
> When building a kernel with CONFIG_PARAVIRT_XXL the paravirt
> infrastructure will always use functions for reading or writing MSRs,
> even when running on bare metal.
> 
> Switch to inline RDMSR/WRMSR instructions in this case, reducing the
> paravirt overhead.
> 
> In order to make this less intrusive, some further reorganization of
> the MSR access helpers is done in the first 5 patches.
> 
> The next 5 patches are converting the non-paravirt case to use direct
> inlining of the MSR access instructions, including the WRMSRNS
> instruction and the immediate variants of RDMSR and WRMSR if possible.
> 
> Patch 11 removes the PV hooks for MSR accesses and implements the
> Xen PV cases via calls depending on X86_FEATURE_XENPV, which results
> in runtime patching those calls away for the non-XenPV case.
> 
> Patch 12 is a final little cleanup patch.
> 
> This series has been tested to work with Xen PV and on bare metal.
> 
> This series is inspired by Xin Li, who used a similar approach, but
> (in my opinion) with some flaws. Originally I thought it should be
> possible to use the paravirt infrastructure, but this turned out to be
> rather complicated, especially for the Xen PV case in the *_safe()
> variants of the MSR access functions.
> 

Looks good to me.

(I'm not at all surprised that paravirt_ops didn't do the job. Both I and Xin
had come to the same conclusion.)


Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>