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

From: David Gibson
Subject: Re: [PATCH] spapr: Fix VSMT mode when it is not supported by the kernel
Date: Tue, 19 Nov 2019 12:00:12 +1100
User-agent: Mutt/1.12.1 (2019-06-15)

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_!

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