RE: [PATCH 0/6] x86/msr: let paravirt inline rdmsr/wrmsr instructions

[Michael Kelley]

From: Juergen Gross <jgross@xxxxxxxx> Sent: Tuesday, May 6, 2025 2:20 AM
> 
> 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 4 patches.
> 
> This series has been tested to work with Xen PV and on bare metal.

I've tested in SEV-SNP and TDX guests with paravisor on Hyper-V. Basic
smoke test showed no issues.

Tested-by: Michael Kelley <mhklinux@xxxxxxxxxxx>

> 
> There has been another approach by Xin Li, which used dedicated #ifdef
> and removing the MSR related paravirt hooks instead of just modifying
> the paravirt code generation.
> 
> Please note that I haven't included the use of WRMSRNS or the
> immediate forms of WRMSR and RDMSR, because I wanted to get some
> feedback on my approach first. Enhancing paravirt for those cases
> is not very complicated, as the main base is already prepared for
> that enhancement.
> 
> This series is based on the x86/msr branch of the tip tree.
> 
> Juergen Gross (6):
>   coco/tdx: Rename MSR access helpers
>   x86/kvm: Rename the KVM private read_msr() function
>   x86/msr: minimize usage of native_*() msr access functions
>   x86/msr: Move MSR trace calls one function level up
>   x86/paravirt: Switch MSR access pv_ops functions to instruction
>     interfaces
>   x86/msr: reduce number of low level MSR access helpers
> 
>  arch/x86/coco/tdx/tdx.c                   |   8 +-
>  arch/x86/hyperv/ivm.c                     |   2 +-
>  arch/x86/include/asm/kvm_host.h           |   2 +-
>  arch/x86/include/asm/msr.h                | 116 ++++++++++-------
>  arch/x86/include/asm/paravirt.h           | 152 ++++++++++++++--------
>  arch/x86/include/asm/paravirt_types.h     |  13 +-
>  arch/x86/include/asm/qspinlock_paravirt.h |   5 +-
>  arch/x86/kernel/kvmclock.c                |   2 +-
>  arch/x86/kernel/paravirt.c                |  26 +++-
>  arch/x86/kvm/svm/svm.c                    |  16 +--
>  arch/x86/kvm/vmx/vmx.c                    |   4 +-
>  arch/x86/xen/enlighten_pv.c               |  60 ++++++---
>  arch/x86/xen/pmu.c                        |   4 +-
>  13 files changed, 262 insertions(+), 148 deletions(-)
> 
> --
> 2.43.0
>