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 [off-list ref] wrote:

On 06/07/21 23:33, Eduardo Habkost wrote:

quoted

On Tue, Jul 6, 2021 at 5:05 PM Paolo Bonzini [off-list ref] wrote:

quoted

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.

quoted

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