On Thu, 2020-08-20 at 17:43 -0700, Sean Christopherson wrote: > On Thu, Aug 20, 2020 at 02:43:56PM -0700, Jim Mattson wrote: > > On Thu, Aug 20, 2020 at 6:34 AM Maxim Levitsky <mlevitsk@xxxxxxxxxx> wrote: > > > This will be used later to return an error when setting this msr fails. > > > > > > For VMX, it already has an error condition when EFER is > > > not in the shared MSR list, so return an error in this case. > > > > > > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx> > > > --- > > > --- a/arch/x86/kvm/x86.c > > > +++ b/arch/x86/kvm/x86.c > > > @@ -1471,7 +1471,8 @@ static int set_efer(struct kvm_vcpu *vcpu, struct msr_data *msr_info) > > > efer &= ~EFER_LMA; > > > efer |= vcpu->arch.efer & EFER_LMA; > > > > > > - kvm_x86_ops.set_efer(vcpu, efer); > > > + if (kvm_x86_ops.set_efer(vcpu, efer)) > > > + return 1; > > > > This seems like a userspace ABI change to me. Previously, it looks > > like userspace could always use KVM_SET_MSRS to set MSR_EFER to 0 or > > EFER_SCE, and it would always succeed. Now, it looks like it will fail > > on CPUs that don't support EFER in hardware. (Perhaps it should fail, > > but it didn't before, AFAICT.) > > KVM emulates SYSCALL, presumably that also works when EFER doesn't exist in > hardware. This is a fair point. How about checking the return value only when '!msr_info->host_initiated' in set_efer? This way userspace initiated EFER write will work as it did before, but guest initiated write will fail (and set_efer already checks and fails for many cases) I also digged a bit around the failure check in VMX, the 'find_msr_entry(vmx, MSR_EFER);' This one if I am not mistaken will only fail when host doesn't support EFER. I don't mind ignoring this error as well as it was before. > > The above also adds weirdness to nested VMX as vmx_set_efer() simply can't > fail. It will now fail on non 64 bit Intel CPUs that support VMX. I do think that we had these for a while. As I said I'll return 0 when find_msr_entry fails, thus return this behavior as it was on Intel. Best regards, Maxim Levitsky
