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

[Laurent Vivier <lvivier@redhat.com>]

On 20/11/2019 05:36, David Gibson wrote:
> On Tue, Nov 19, 2019 at 04:45:26PM +0100, Greg Kurz wrote:
>> On Tue, 19 Nov 2019 15:06:51 +0100
>> Laurent Vivier <lvivier@redhat.com> wrote:
>>
>>> 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
>>
>> Ohhh... it isn't possible to initialize Qtest with machine specific
>> properties ? That's a bit unfortunate :-\
> 
> Uhh... I don't see why we can't.  Couldn't we just put either -machine
> vsmt=8 or -smp 8 into the cmd_src / cmd_dst printfs() in the
> strcmp(arch, "ppc64") case?

Yes, but we need to do that to all other tests that fail. test-migration
is not the only one impacted by the problem (we have also pxe-test), so
it's why I thought to fix the problem in a generic place.

But it seems there are only this couple of tests that are impacted so I
can modify both instead. I think only tests that really start CPU have
the problem.

I'm going to send a patch to fix that.

Thanks,
Laurent