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

[Philippe Mathieu-Daudé]

On 3/18/21 12:21 PM, Peter Maydell wrote:
> On Thu, 18 Mar 2021 at 11:09, Philippe Mathieu-Daudé <philmd@redhat.com> 
> wrote:
>> Still, I'm not sure it makes sense. If you want to migrate a such
>> machine, KVM can not virtualize it, so you'll be forced to use TCG
>> right? In that case cpu_tcg is built in and you have the A15.
>>
>> IOW the problem is not this patch, it is that since 82bf7ae84ce we
>> can not migrate A15.
> 
> Do you mean "we can't migrate a TCG A15" ? That would be a problem.

No problem here.

> Or do you mean "we can't migrate a KVM A15" ? That's entirely
> expected when we drop support for KVM A15 :-)

Yes, this is why I mentioned this is mostly a problem for downstream
distributions.

> Or do you mean "migration from KVM to TCG doesn't work?" That's
> a pre-existing thing I don't expect to work (we don't put anything
> in to stop it working, but I'm pretty sure there will be a bunch
> of things that mean it doesn't in practice work.)
> 
>> So we need both 1/ revert 82bf7ae84ce and 2/ be sure the kernel
>> support 32-bit host... Am I missing something?
> 
> We're definitely not reverting 82bf7ae84ce if we can avoid it...

I tend to agree. The problem is for the running VMs started before
82bf7ae84ce (so up to any fork based on v5.2). I don't know what
the forks are supposed to do with the running VMs if they want to
migrate them to newer QEMU (or upgrade the host QEMU).

I don't expect a guest being happy and reliable if we underneath
change its CPU by another one while migrating it...

KVM to TCG is not tested much indeed, and I don't think the performance
impact will be well accepted :)