* Andy Lutomirski <luto@xxxxxxxxxx> wrote: > Setting CONFIG_PARAVIRT=y has an unintended side effect: it silently > turns all rdmsr and wrmsr operations into the safe variants without > any checks that the operations actually succeed. > > This is IMO awful: it papers over bugs. In particular, KVM gueests > might be unwittingly depending on this behavior because > CONFIG_KVM_GUEST currently depends on CONFIG_PARAVIRT. I'm not > aware of any such problems, but applying this series would be a good > way to shake them out. > > Fix it so that the MSR operations work the same on CONFIG_PARAVIRT=n > and CONFIG_PARAVIRT=y as long as Xen isn't being used. The Xen > maintainers are welcome to make a similar change on top of this. > > Since there's plenty of time before the next merge window, I think > we should apply and fix anything that breaks. No, I think we should at most generate a warning instead, and not crash the kernel via rdmsr()! Most big distro kernels on bare metal have CONFIG_PARAVIRT=y (I checked Ubuntu and Fedora), so we are potentially exposing a lot of users to problems. Crashing the bootup on an unknown MSR is bad. Many MSR reads and writes are non-critical and returning the 'safe' result is much better than crashing or hanging the bootup. ( We should double check that rdmsr()/wrmsr() results are never left uninitialized, but are set to zero or so, for cases where the return code is not checked. ) Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
