Re: [PATCH v3 00/16] x86/msr: Inline rdmsr/wrmsr instructions

["Jürgen Groß" <jgross@suse.com>]

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On 18.02.26 21:37, H. Peter Anvin wrote:
> On February 18, 2026 12:21:17 AM PST, Juergen Gross <jgross@suse.com> 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.
>>
>> The first patch is a prerequisite fix for alternative patching. Its
>> is needed due to the initial indirect call needs to be padded with
>> NOPs in some cases with the following patches.
>>
>> In order to make this less intrusive, some further reorganization of
>> the MSR access helpers is done in the patches 1-6.
>>
>> The next 4 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.
>>
>> Patches 11-13 are some further preparations for making the real switch
>> to directly patch in the native MSR instructions easier.
>>
>> Patch 14 is switching the paravirt MSR function interface from normal
>> call ABI to one more similar to the native MSR instructions.
>>
>> Patch 15 is a little cleanup patch.
>>
>> Patch 16 is the final step for patching in the native MSR instructions
>> when not running as a Xen PV guest.
>>
>> This series has been tested to work with Xen PV and on bare metal.
>>
>> Note that there is more room for improvement. This series is sent out
>> to get a first impression how the code will basically look like.
> 
> Does that mean you are considering this patchset an RFC? If so, you should put that in the subject header.

It is one possible solution.

> 
>> Right now the same problem is solved differently for the paravirt and
>> the non-paravirt cases. In case this is not desired, there are two
>> possibilities to merge the two implementations. Both solutions have
>> the common idea to have rather similar code for paravirt and
>> non-paravirt variants, but just use a different main macro for
>> generating the respective code. For making the code of both possible
>> scenarios more similar, the following variants are possible:
>>
>> 1. Remove the micro-optimizations of the non-paravirt case, making
>>    it similar to the paravirt code in my series. This has the
>>    advantage of being more simple, but might have a very small
>>    negative performance impact (probably not really detectable).
>>
>> 2. Add the same micro-optimizations to the paravirt case, requiring
>>    to enhance paravirt patching to support a to be patched indirect
>>    call in the middle of the initial code snipplet.
>>
>> In both cases the native MSR function variants would no longer be
>> usable in the paravirt case, but this would mostly affect Xen, as it
>> would need to open code the WRMSR/RDMSR instructions to be used
>> instead the native_*msr*() functions.
>>
>> Changes since V2:
>> - switch back to the paravirt approach
>>
>> Changes since V1:
>> - Use Xin Li's approach for inlining
>> - Several new patches
>>
>> Juergen Gross (16):
>>   x86/alternative: Support alt_replace_call() with instructions after
>>     call
>>   coco/tdx: Rename MSR access helpers
>>   x86/sev: Replace call of native_wrmsr() with native_wrmsrq()
>>   KVM: x86: Remove 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/opcode: Add immediate form MSR instructions
>>   x86/extable: Add support for immediate form MSR instructions
>>   x86/msr: Use the alternatives mechanism for WRMSR
>>   x86/msr: Use the alternatives mechanism for RDMSR
>>   x86/alternatives: Add ALTERNATIVE_4()
>>   x86/paravirt: Split off MSR related hooks into new header
>>   x86/paravirt: Prepare support of MSR instruction interfaces
>>   x86/paravirt: Switch MSR access pv_ops functions to instruction
>>     interfaces
>>   x86/msr: Reduce number of low level MSR access helpers
>>   x86/paravirt: Use alternatives for MSR access with paravirt
>>
>> arch/x86/coco/sev/internal.h              |   7 +-
>> arch/x86/coco/tdx/tdx.c                   |   8 +-
>> arch/x86/hyperv/ivm.c                     |   2 +-
>> arch/x86/include/asm/alternative.h        |   6 +
>> arch/x86/include/asm/fred.h               |   2 +-
>> arch/x86/include/asm/kvm_host.h           |  10 -
>> arch/x86/include/asm/msr.h                | 345 ++++++++++++++++------
>> arch/x86/include/asm/paravirt-msr.h       | 148 ++++++++++
>> arch/x86/include/asm/paravirt.h           |  67 -----
>> arch/x86/include/asm/paravirt_types.h     |  57 ++--
>> arch/x86/include/asm/qspinlock_paravirt.h |   4 +-
>> arch/x86/kernel/alternative.c             |   5 +-
>> arch/x86/kernel/cpu/mshyperv.c            |   7 +-
>> arch/x86/kernel/kvmclock.c                |   2 +-
>> arch/x86/kernel/paravirt.c                |  42 ++-
>> arch/x86/kvm/svm/svm.c                    |  16 +-
>> arch/x86/kvm/vmx/tdx.c                    |   2 +-
>> arch/x86/kvm/vmx/vmx.c                    |   8 +-
>> arch/x86/lib/x86-opcode-map.txt           |   5 +-
>> arch/x86/mm/extable.c                     |  35 ++-
>> arch/x86/xen/enlighten_pv.c               |  52 +++-
>> arch/x86/xen/pmu.c                        |   4 +-
>> tools/arch/x86/lib/x86-opcode-map.txt     |   5 +-
>> tools/objtool/check.c                     |   1 +
>> 24 files changed, 576 insertions(+), 264 deletions(-)
>> create mode 100644 arch/x86/include/asm/paravirt-msr.h
>>
> 
> Could you clarify *on the high design level* what "go back to the paravirt approach" means, and the motivation for that?

This is related to V2 of this series, where I used a static branch for
special casing Xen PV.

Peter Zijlstra commented on that asking to try harder using the pv_ops
hooks for Xen PV, too.

> Note that for Xen *most* MSRs fall in one of two categories: those that are dropped entirely and those that are just passed straight on to the hardware.
> 
> I don't know if anyone cares about optimizing PV Xen anymore, but at least in theory Xen can un-paravirtualize most sites.

The problem with that is, that this would need to be taken care at the
callers' sites, "poisoning" a lot of code with Xen specific paths. Or we'd
need to use the native variants explicitly at all places where Xen PV
would just use the MSR instructions itself. But please be aware, that
there are plans to introduce a hypercall for Xen to speed up MSR accesses,
which would reduce the "passed through to hardware" cases to 0.


Juergen

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