Re: [RFC PATCH 2/3] tests/qtest/migration: Only test aarch64 on TCG

[Peter Maydell <peter.maydell@linaro.org>]

On Wed, 30 Jul 2025 at 21:52, Fabiano Rosas <farosas@suse.de> wrote:
>
> Currently our aarch64 tests are only being run using identical QEMU
> versions. When running the tests with different QEMU versions, which
> is a common use-case for migration, the tests are broken due to the
> current choice of the 'max' cpu, which is not stable and is prone to
> breaking migration.
>
> This means aarch64 tests are currently only testing about the same
> situations as any other arch, i.e. no arm-specific testing is being
> done.
>
> To make the aarch64 tests more useful, -cpu max will be changed to
> -cpu neoverse-n1 in the next patch. Before doing that, make sure
> aarch64 tests only run with TCG, since KVM testing depends on usage of
> the -cpu host and we currently don't have code to switch between cpus
> according to test runtime environment.
>
> Also, TCG alone should allow us to catch most issues with migration,
> since there is no guarantee of a uniform environment as there is with
> KVM.

The difficulty with only testing TCG migration is that now
we're testing the setup that most cross-versions migration users
don't care about. At least my assumption is that it's KVM
cross-version migration that is the real use case here.

For instance, this migration bug with the DBGDTR register
isn't a problem for KVM, because with KVM we use the kernel
to tell us what system registers are present, and whether
a register is defined with a cpreg in QEMU or not doesn't
affect what we put on the wire for migration. Conversely
there might be migration compat issues that show up only
with KVM and not TCG (though the most obvious source of those
would be host kernel changes, which is kind of out of scope
for us).

Though of course with our CI jobs we're probably not
doing AArch64 KVM cross-version testing anyway...

thanks
-- PMM