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AJvYcCUhxcvZk47E96/weIYU4i6KUX4RWhaQlThjlImvJPm4Y0p0XaOIrV3Pz9viB8JEa6jp6pR2KUOx2c2RNmk=@vger.kernel.org X-Gm-Message-State: AOJu0YwzVX2gyjK09S5WG3OkYGgf+SMFV+r0M9IhNCx/TZMYfKRdhzQB LKi/1yrUrohy6heQYS0RoiT/p/N/4g2w8twzb3rKzUe1ax2TTxkIrJND/w0Y7zPD7m2TfK2CjGE aWeLx5Q== X-Received: from plzt6.prod.google.com ([2002:a17:902:bc46:b0:2ae:abe9:b38b]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:903:1503:b0:2af:c7c:afbc with SMTP id d9443c01a7336-2af0c7cb19amr22428855ad.31.1773432906364; Fri, 13 Mar 2026 13:15:06 -0700 (PDT) Date: Fri, 13 Mar 2026 13:15:04 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260310220414.2569208-1-seanjc@google.com> Message-ID: Subject: Re: [PATCH] KVM: SVM: Add a helper to get LBR field pointer to dedup MSR accesses From: Sean Christopherson To: Yosry Ahmed Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable On Fri, Mar 13, 2026, Yosry Ahmed wrote: > On Tue, Mar 10, 2026 at 3:04=E2=80=AFPM Sean Christopherson 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 > > Signed-off-by: Sean Christopherson > > --- > > 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 *dat= a) > > 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(); > > +} >=20 > This actually fires for me with clang (from both callsites): >=20 > /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/linu= x/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/linu= x/build_bug.h:39:37: > note: expanded from macro 'BUILD_BUG_ON_MSG' > 39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), m= sg) > | ^ > /usr/local/google/home/yosryahmed/dev/kernel.org/nsvm_fixups/include/linu= x/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/linu= x/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/linu= x/compiler_types.h:687:4: > note: expanded from macro '__compiletime_assert' > 687 | prefix ## suffix(); > \ > | ^ > :102:1: note: expanded from here > 102 | __compiletime_assert_1058 > | ^ Hrm, I was assuming the compiler would be smart to understand the other cas= es are unreachable. What clang version, and are you doing anything special wi= th your config? Even with KASAN and other sanitizers enabled, clang-19 is a-o= k with the code. And I haven't gotten yelled at by build bots (yet).