Re: [Qemu-devel] [PATCH for-2.11.2] spapr: make pseries-2.11 the default machine type

[David Gibson]

On Wed, Jun 20, 2018 at 09:27:27AM +0200, Greg Kurz wrote:
> On Wed, 20 Jun 2018 11:22:40 +1000
> David Gibson <address@hidden> wrote:
> 
> > On Tue, Jun 19, 2018 at 01:11:28PM +0200, Greg Kurz wrote:
> > > On Mon, 18 Jun 2018 21:04:38 -0500
> > > Michael Roth <address@hidden> wrote:
> > >   
> > > > Quoting Greg Kurz (2018-05-22 12:17:28)  
> > > > > The spapr capability framework was introduced in QEMU 2.12. It allows
> > > > > to have an explicit control on how host features are exposed to the
> > > > > guest. This is especially needed to handle migration between hetero-
> > > > > geneous hosts (eg, POWER8 to POWER9). It is also used to expose fixes/
> > > > > workarounds against speculative execution vulnerabilities to guests.
> > > > > The framework was hence backported to QEMU 2.11.1, especially these
> > > > > commits:
> > > > > 
> > > > > 0fac4aa93074 spapr: Add pseries-2.12 machine type
> > > > > 9070f408f491 spapr: Treat Hardware Transactional Memory (HTM) as an
> > > > >  optional capability
> > > > > 
> > > > > 0fac4aa93074 has the confusing effect of making pseries-2.12 the 
> > > > > default
> > > > > machine type for QEMU 2.11.1, instead of the expected pseries-2.11. 
> > > > > This
> > > > > patch changes the default machine back to pseries-2.11.
> > > > > 
> > > > > Unfortunately, 9070f408f491 enforces the HTM capability for 
> > > > > pseries-2.11.
> > > > > This isn't supported by TCG and breaks 'make check'. So this patch 
> > > > > also
> > > > > adds a hack to turn HTM off when using TCG.    
> > > > 
> > > > I noticed this ends up breaking TCG migration for 2.11.2 -> 2.12, I
> > > > get this on the target side even when specifying -machine
> > > > pseries-2.11,cap-htm=off for both ends:
> > > > 
> > > >   qemu-system-ppc64: cap-htm higher level (1) in incoming stream than 
> > > > on destination (0)
> > > >   qemu-system-ppc64: error while loading state for instance 0x0 of 
> > > > device 'spapr'
> > > >   qemu-system-ppc64: load of migration failed: Invalid argument
> > > > 
> > > > I'm not sure we care all that much about it but it's a regression from 
> > > > 2.11.1
> > > > at least. The main issue seems to be the default caps for 2.11.2 for 
> > > > TCG are
> > > > now different from 2.11 and 2.12+, but spapr_cap_##cap##_needed still 
> > > > assumes
> > > > everything is the same across all these versions and as such opts not to
> > > > migrate cap-htm=off, since that's the default for 2.11.2. This results 
> > > > in the
> > > > target assuming the source was using the default, which is cap-htm=on,
> > > > and since that disagrees with the spapr->eff we get a failure.
> > > > 
> > > > It seems spapr_cap_##cap##_needed needs to be fixed up to address that,
> > > > but I'm not sure how best to deal with backward compatibility in that 
> > > > case.
> > > > Any ideas? If it ends up being a trade-off I think forward 
> > > > compatibility is
> > > > more important.
> > > >   
> > > 
> > > Yeah, we shouldn't change the default since it affects the migration 
> > > logic :-\
> > > 
> > > The motivation behind this hack is to fix TCG based 'make check', because
> > > it doesn't pass cap-htm=off, and thus can't run with pseries-2.11.
> > > 
> > > Another possibility is to let the default as is, and to disable HTM after 
> > > the
> > > default caps have been applied.
> > > 
> > > Something like that squashed into this patch:
> > > 
> > > diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
> > > index 82043e60e78b..26e6be043b18 100644
> > > --- a/hw/ppc/spapr_caps.c
> > > +++ b/hw/ppc/spapr_caps.c
> > > @@ -285,11 +285,6 @@ static sPAPRCapabilities 
> > > default_caps_with_cpu(sPAPRMachineState *spapr,
> > >  
> > >      caps = smc->default_caps;
> > >  
> > > -    /* HACK for 2.11.2: fix make check */
> > > -    if (tcg_enabled()) {
> > > -        caps.caps[SPAPR_CAP_HTM] = SPAPR_CAP_OFF;
> > > -    }
> > > -
> > >      if (!ppc_check_compat(cpu, CPU_POWERPC_LOGICAL_2_07,
> > >                            0, spapr->max_compat_pvr)) {
> > >          caps.caps[SPAPR_CAP_HTM] = SPAPR_CAP_OFF;
> > > @@ -405,6 +400,11 @@ void spapr_caps_reset(sPAPRMachineState *spapr)
> > >          }
> > >      }
> > >  
> > > +    /* HACK for 2.11.2: fix make check */
> > > +    if (tcg_enabled()) {
> > > +        spapr->eff.caps[SPAPR_CAP_HTM] = SPAPR_CAP_OFF;
> > > +    }
> > > +
> > >      /* .. then apply those caps to the virtual hardware */
> > >  
> > >      for (i = 0; i < SPAPR_CAP_NUM; i++) {
> > > ---------------------  
> > 
> > Nooooo!  The whole point of the caps stuff is to stop changing guest
> > visible behaviours based on host side configuration like the
> > accelerator.  So, really, let's not put it back in.
> > 
> 
> Yes I'm aware of all the work we're doing to hide host details.
> I was just thinking this small violation would be an acceptable
> tradeoff.

The reason I wrote the caps stuff was that I kept finding that
supposedly small violations of this still kept biting us in the arse.

> > The correct fix is to add cap-htm=off to the testcases.  Gross, but
> > necessary.
> > 
> 
> Ok, then I'll try to come up with patches ASAP so that Mike can release
> 2.11.2.

After I sent that I did think of some alternatives.  We can't change
the behaviour based on the accelerator, but we might be able to change
it based on qtest_enabled().


-- 
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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