On Fri, Sep 16, 2022 at 1:14 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > On Thu, Sep 15, 2022, Jim Mattson wrote: > > KVM has never properly virtualized EFER.LMSLE. When the "nested" > > module parameter is true, it allows an SVM guest to set EFER.LMSLE, > > and it passes the bit through in the VMCB, but the KVM emulator > > doesn't perform the required data segment limit checks in 64-bit mode. > > > > With Zen3, AMD has dropped support for EFER.LMSLE. Hence, if a Zen3 > > guest sets EFER.LMSLE, the next VMRUN will fail with "invalid VMCB." > > > > When the host reports X86_FEATURE_NO_LMSLE, treat EFER.LMSLE as a > > reserved bit in the guest. Now, if a guest tries to set EFER.LMSLE on > > a host without support for EFER.LMSLE, the WRMSR will raise a #GP. > > > > At the moment, the #GP may come as a surprise, but it's an improvement > > over the failed VMRUN. The #GP will be vindicated anon. > > > > Fixes: eec4b140c924 ("KVM: SVM: Allow EFER.LMSLE to be set with nested svm") > > Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx> > > --- > > arch/x86/kvm/svm/svm.c | 4 +++- > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c > > index f3813dbacb9f..7c4fd594166c 100644 > > --- a/arch/x86/kvm/svm/svm.c > > +++ b/arch/x86/kvm/svm/svm.c > > @@ -5012,7 +5012,9 @@ static __init int svm_hardware_setup(void) > > > > if (nested) { > > printk(KERN_INFO "kvm: Nested Virtualization enabled\n"); > > - kvm_enable_efer_bits(EFER_SVME | EFER_LMSLE); > > + kvm_enable_efer_bits(EFER_SVME); > > + if (!boot_cpu_has(X86_FEATURE_NO_LMSLE)) > > + kvm_enable_efer_bits(EFER_LMSLE); > > Since KVM doesn't correctly virtualize EFER.LMSLE, I wonder if we can get away with > dropping support entirely. I.e. delete the reference to EFER_LMSLE and unconditionally > set F(NO_LMSLE) in KVM_GET_SUPPORTED_CPUID. It's possible that SLES11 Xen 4.0 sets the bit, but never actually uses truncated segments in 64-bit mode. In any case, according to the original commit, it won't boot if setting EFER.LMSLE is not allowed.