Re: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE

On 06/07/21 23:33, Eduardo Habkost wrote: > On Tue, Jul 6, 2021 at 5:05 PM Paolo Bonzini <pbonzini(a)redhat.com&gt; 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.