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

From: Laurent Vivier
Subject: Re: [PATCH] spapr: Fix VSMT mode when it is not supported by the kernel
Date: Wed, 20 Nov 2019 12:28:19 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2

On 20/11/2019 10:00, Laurent Vivier wrote:
> 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 <address@hidden> 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 <address@hidden> 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.

And again, it's a little bit more complicated than expected: setting
vsmt to 8 works only with kvm_hv, but breaks in case of TCG or kvm_pr.
So the test must check what is in use...


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