> Am 19.08.2020 um 23:27 schrieb Jim Mattson <jmattson@xxxxxxxxxx>: > >> On Mon, Aug 3, 2020 at 2:14 PM Alexander Graf <graf@xxxxxxxxxx> wrote: >> >> While tying to add support for the MSR_CORE_THREAD_COUNT MSR in KVM, >> I realized that we were still in a world where user space has no control >> over what happens with MSR emulation in KVM. >> >> That is bad for multiple reasons. In my case, I wanted to emulate the >> MSR in user space, because it's a CPU specific register that does not >> exist on older CPUs and that really only contains informational data that >> is on the package level, so it's a natural fit for user space to provide >> it. >> >> However, it is also bad on a platform compatibility level. Currrently, >> KVM has no way to expose different MSRs based on the selected target CPU >> type. >> >> This patch set introduces a way for user space to indicate to KVM which >> MSRs should be handled in kernel space. With that, we can solve part of >> the platform compatibility story. Or at least we can not handle AMD specific >> MSRs on an Intel platform and vice versa. >> >> In addition, it introduces a way for user space to get into the loop >> when an MSR access would generate a #GP fault, such as when KVM finds an >> MSR that is not handled by the in-kernel MSR emulation or when the guest >> is trying to access reserved registers. >> >> In combination with the allow list, the user space trapping allows us >> to emulate arbitrary MSRs in user space, paving the way for target CPU >> specific MSR implementations from user space. > > This is somewhat misleading. If you don't modify the MSR permission > bitmaps, as Aaron has done, you cannot emulate *arbitrary* MSRs in > userspace. You can only emulate MSRs that kvm is going to intercept. > Moreover, since the set of intercepted MSRs evolves over time, this > isn't a stable API. Yeah, I wrote up a patch today to do the passthrough bitmap masking dynamically, partly based on Aaron's patches. I have not verified it yet though and will need to clean up SVM as well. I'll see how far I get with it for the rest of the week. Special MSRs like EFER also irritate me a bit. We can't really trap on them - most code paths just know they're handled in kernel. Maybe I'll add some sanity checks as well... Alex Amazon Development Center Germany GmbH Krausenstr. 38 10117 Berlin Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B Sitz: Berlin Ust-ID: DE 289 237 879
