From: Sean Christopherson Date: Thu, 13 Nov 2025 23:14:17 +0000 (-0800) Subject: KVM: SVM: Skip OSVW MSR reads if KVM is treating all errata as present X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=089af84641b574990da97d4674706a0303abca34;p=thirdparty%2Flinux.git KVM: SVM: Skip OSVW MSR reads if KVM is treating all errata as present Don't bother reading the OSVW MSRs if osvw_len is already zero, i.e. if KVM is already treating all errata as present, in which case the positive path of the if-statement is one giant nop. Opportunistically update the comment to more thoroughly explain how the MSRs work and why the code does what it does. Link: https://patch.msgid.link/20251113231420.1695919-3-seanjc@google.com Signed-off-by: Sean Christopherson --- diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 9d611bfbcc8c2..695cebf8d7241 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -543,15 +543,25 @@ static int svm_enable_virtualization_cpu(void) /* - * Get OSVW bits. + * Get OS-Visible Workarounds (OSVW) bits. * * Note that it is possible to have a system with mixed processor * revisions and therefore different OSVW bits. If bits are not the same * on different processors then choose the worst case (i.e. if erratum * is present on one processor and not on another then assume that the * erratum is present everywhere). + * + * Note #2! The OSVW MSRs are used to communciate that an erratum is + * NOT present! Software must assume erratum as present if its bit is + * set in OSVW_STATUS *or* the bit number exceeds OSVW_ID_LENGTH. If + * either RDMSR fails, simply zero out the length to treat all errata + * as being present. Similarly, use the *minimum* length across all + * CPUs, not the maximum length. + * + * If the length is zero, then is KVM already treating all errata as + * being present and there's nothing left to do. */ - if (cpu_has(&boot_cpu_data, X86_FEATURE_OSVW)) { + if (osvw_len && cpu_has(&boot_cpu_data, X86_FEATURE_OSVW)) { u64 len, status = 0; int err;