x86/msr: let paravirt inline rdmsr/wrmsr instructions

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

Posted by Juergen Gross 5 months, 4 weeks ago

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.

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

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

Posted by Michael Kelley 5 months, 3 weeks ago

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
>