Re: [PATCH] spapr: Fix VSMT mode when it is not supported by the kernel

[Laurent Vivier <lvivier@redhat.com>]

On 19/11/2019 02:00, David Gibson wrote:
> On Fri, Nov 08, 2019 at 05:47:59PM +0100, Greg Kurz wrote:
>> On Fri,  8 Nov 2019 16:40:35 +0100
>> Laurent Vivier <lvivier@redhat.com> wrote:
>>
>>> Commit 29cb4187497d sets by default the VSMT to smp_threads,
>>> but older kernels (< 4.13) don't support that.
>>>
>>> We can reasonably restore previous behavior with this kernel
>>> to allow to run QEMU as before.
>>>
>>> If VSMT is not supported, VSMT will be set to MAX(8, smp_threads)
>>> as it is done for previous machine types (< pseries-4.2)
>>>
>>
>> It is usually _bad_ to base the machine behavior on host capabilities.
>> What happens if we migrate between an older kernel and a recent one ?
> 
> Right.  We're really trying to remove instaces of such behaviour.  I'd
> prefer to completely revert Greg's original patch than to re-introduce
> host configuration dependency into the guest configuration..
> 
>> I understand this is to fix tests/migration-test on older kernels.
>> Couldn't this be achieved with migration-test doing some introspection
>> and maybe pass vsmt=8 on the QEMU command line ?
> 
> ..adjusting the test case like this might be a better idea, though.
> 
> What's the test setup where we're using the old kernel?  I really only
> applied the original patch on the guess that we didn't really care
> about kernels that old.  The fact you've hit this in practice makes me
> doubt that assumption.
> 

The way to fix the tests is to add "-smp threads=8" on the command line
(for all tests, so basically in qtest_init_without_qmp_handshake(), and
it will impact all the machine types), and we have to check if it is
ppc64/pseries to do that, and there it becomes a little bit complicated
for a so small piece of code.

So I think the best to do is to revert Greg's patch.

Thanks,
Laurent