Re: [PATCH v2 3/3] target/arm: Restrict v7A TCG cpus to TCG accel

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Hi Peter,

On 3/11/21 11:43 AM, Claudio Fontana wrote:
> On 2/21/21 11:26 PM, Philippe Mathieu-Daudé wrote:
>> KVM requires the target cpu to be at least ARMv8 architecture
>> (support on ARMv7 has been dropped in commit 82bf7ae84ce:
>> "target/arm: Remove KVM support for 32-bit Arm hosts").
>>
>> A KVM-only build won't be able to run TCG cpus, move the
>> v7A CPU definitions to cpu_tcg.c.
>>
>> Reported-by: Peter Maydell <peter.maydell@linaro.org>
>> Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
>> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> 
> 
> Here I think it's better to keep the "a15" cpu around,
> until we fix the board configuration situation.
> 
> I added a patch that does that into my KVM-only build series, to avoid the resulting breakages.

Actually I got a downstream report that this break migration from QEMU
5.2 to QEMU 6.0.

I first thought it was on an old kernel (with 32-bit KVM enabled),
but not, it is for Aarch64 VMs on recent KVM (without 32-bit support)
but the 'virt' machines were started with default Cortex-A15 CPU...

    mc->default_cpu_type = ARM_CPU_TYPE_NAME("cortex-a15");

I'm not sure upstream should care about this case, but I though
maybe you could give me hints about the best way to keep old VMs
working, as this likely affects any distribution. Obviously not
upgrading QEMU is not a solution :)

The simplest seems to revert 82bf7ae84ce and this patch, but I
doubt this will be enough.

Maybe there is some clever thing to do before reverting 82bf7ae84ce,
that could also benefit upstream, by doing something with versioned
machines? I have no idea (yet) how that work and if it could work.

Thanks,

Phil.