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

[David Gibson]

On Wed, Nov 20, 2019 at 03:28:19PM +0100, Laurent Vivier wrote:
> On 20/11/2019 12:41, David Gibson wrote:
> > 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.
> 
> In I was mistakenly using "-smp threads=8", with "-M vsmt=8" it works
> with TCG and KVM PR (with a warning).

Ah, yes, that's not so bad.

-- 
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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