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. Signed-off-by: Sean Christopherson --- arch/x86/kvm/svm/svm.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 5612e46e481c..0101da1a3c26 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -539,15 +539,25 @@ 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; -- 2.52.0.rc1.455.g30608eb744-goog