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

Michael Kelley
2025-05-10T16:03:58Z

From: Juergen Gross <jgross@suse.com> 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@outlook.com>

>
> 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
>