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.
Paolo
