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

[David Gibson]

On Wed, Nov 20, 2019 at 12:28:19PM +0100, Laurent Vivier wrote:
> 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...

Ugh, yeah, that's getting too ugly.  I think the feasible options are
either to revert the patch, or just say that upstream qemu no longer
supports a RHEL7 host.

-- 
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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