On Wed, Jul 7, 2021 at 8:09 AM Eduardo Habkost <ehabkost@xxxxxxxxxx> wrote: > > 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. Has there ever been a clear definition? Family, model, and stepping, for instance: are these the only values supported? That would make cross-platform migration impossible. What about the vendor string? Is that the only value supported? That would make cross-vendor migration impossible. For the maximum input value for basic CPUID information (CPUID.0H:EAX), is that the only value supported, or is it the maximum value supported? On the various individual feature bits, does a '1' imply that '0' is also supported, or is '1' the only value supported? What about the feature bits with reversed polarity (e.g. CPUID.(EAX=07H,ECX=0):EBX.FDP_EXCPTN_ONLY[bit 6])? This API has never made sense to me. I have no idea how to interpret what it is telling me. > 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 >
