CCing libvir-list, Jiri Denemark, Michal Privoznik, so they are aware that the definition of "supported CPU features" will probably become a bit more complex in the future. On Tue, Jul 6, 2021 at 5:58 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > On 06/07/21 23:33, Eduardo Habkost wrote: > > On Tue, Jul 6, 2021 at 5:05 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > >> It's a bit tricky, because HLE and RTM won't really behave well. An old > >> guest that sees RTM=1 might end up retrying and aborting transactions > >> too much. So I'm not sure that a QEMU "-cpu host" guest should have HLE > >> and RTM enabled. > > > > Is the purpose of GET_SUPPORTED_CPUID to return what is supported by > > KVM, or to return what "-cpu host" should enable by default? They are > > conflicting requirements in this case. > > In theory there is GET_EMULATED_CPUID for the former, so it should be > the latter. In practice neither QEMU nor Libvirt use it; maybe now we > have a good reason to add it, but note that userspace could also check > host RTM_ALWAYS_ABORT. > > > Returning HLE=1,RTM=1 in GET_SUPPORTED_CPUID makes existing userspace > > take bad decisions until it's updated. > > > > Returning HLE=0,RTM=0 in GET_SUPPORTED_CPUID prevents existing > > userspace from resuming existing VMs (despite being technically > > possible). > > > > The first option has an easy workaround that doesn't require a > > software update (disabling HLE/RTM in the VM configuration). The > > second option doesn't have a workaround. I'm inclined towards the > > first option. > > The default has already been tsx=off for a while though, so checking > either GET_EMULATED_CPUID or host RTM_ALWAYS_ABORT in userspace might > also be feasible for those that are still on tsx=on. This sounds like a perfect use case for GET_EMULATED_CPUID. My only concern is breaking existing userspace. But if this was already broken for a few kernel releases due to tsx=off being the default, maybe GET_EMULATED_CPUID will be a reasonable approach. -- Eduardo
