Re: [PATCH] KVM: SVM: Add a helper to get LBR field pointer to dedup MSR accesses

[Yosry Ahmed]

On Tue, Mar 10, 2026 at 3:04 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> Add a helper to get a pointer to the corresponding VMCB field given an LBR
> MSR index, and use it to dedup the handling in svm_{g,s}et_msr().
>
> No functional change intended.
>
> Suggested-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/svm.c | 48 +++++++++++++++++-------------------------
>  1 file changed, 19 insertions(+), 29 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 3407deac90bd..88999b6fc8a3 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2750,6 +2750,23 @@ static int svm_get_feature_msr(u32 msr, u64 *data)
>         return 0;
>  }
>
> +static __always_inline u64 *svm_vmcb_lbr(struct vcpu_svm *svm, u32 msr)
> +{
> +       switch (msr) {
> +       case MSR_IA32_LASTBRANCHFROMIP:
> +               return &svm->vmcb->save.br_from;
> +       case MSR_IA32_LASTBRANCHTOIP:
> +               return &svm->vmcb->save.br_to;
> +       case MSR_IA32_LASTINTFROMIP:
> +               return &svm->vmcb->save.last_excp_from;
> +       case MSR_IA32_LASTINTTOIP:
> +               return &svm->vmcb->save.last_excp_to;
> +       default:
> +               break;
> +       }
> +       BUILD_BUG();
> +}

This actually fires for me with clang (from both callsites):

/usr/local/google/home/yosryahmed/dev/kernel.org/nsvm_fixups/arch/x86/kvm/svm/svm.c:2766:2:
error: call to '__compiletime_assert_1058' declared with 'error'
attribute: BUILD_BUG failed
 2766 |         BUILD_BUG();
      |         ^
/usr/local/google/home/yosryahmed/dev/kernel.org/nsvm_fixups/include/linux/build_bug.h:59:21:
note: expanded from macro 'BUILD_BUG'
   59 | #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
      |                     ^
/usr/local/google/home/yosryahmed/dev/kernel.org/nsvm_fixups/include/linux/build_bug.h:39:37:
note: expanded from macro 'BUILD_BUG_ON_MSG'
   39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
      |                                     ^
/usr/local/google/home/yosryahmed/dev/kernel.org/nsvm_fixups/include/linux/compiler_types.h:706:2:
note: expanded from macro 'compiletime_assert'
  706 |         _compiletime_assert(condition, msg,
__compiletime_assert_, __COUNTER__)
      |         ^
/usr/local/google/home/yosryahmed/dev/kernel.org/nsvm_fixups/include/linux/compiler_types.h:694:2:
note: expanded from macro '_compiletime_assert'
  694 |         __compiletime_assert(condition, msg, prefix, suffix)
      |         ^
/usr/local/google/home/yosryahmed/dev/kernel.org/nsvm_fixups/include/linux/compiler_types.h:687:4:
note: expanded from macro '__compiletime_assert'
  687 |                         prefix ## suffix();
         \
      |                         ^
<scratch space>:102:1: note: expanded from here
  102 | __compiletime_assert_1058
      | ^