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

[Laurent Vivier]

On 08/11/2019 17:47, 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.

This is already the case, statically: your patch guesses the kernel
always support VSMT. So you can't start the machine (and thus can't
migrate it to/from).

> What happens if we migrate between an older kernel and a recent one ?

I think migration is supported correctly only if parameters are
explicitly set. So this is not our case.

> 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 ?

It could be a little bit complicated to instrospect this.
We could also set by default vsmt=8 at the test level.

Thanks,
Laurent