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

[David Gibson]

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.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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