Re: [Qemu-devel] [PATCH v2 for-2.11.2] spapr: make pseries-2.11 the default machine type

[David Gibson]

On Thu, Jun 21, 2018 at 03:23:21PM +0200, Greg Kurz wrote:
> On Thu, 21 Jun 2018 11:18:09 +1000
> David Gibson <address@hidden> wrote:
> 
> > On Wed, Jun 20, 2018 at 02:54:15PM +0200, Greg Kurz wrote:
> > > 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
> > > to be enabled by default, ie, when not passing cap-htm on the command
> > > line. This breaks several 'make check' testcases that run 
> > > qemu-system-ppc64
> > > with TCG.
> > > 
> > > The only sane way to fix this is to adapt the impacted testcases so that
> > > they all pass cap-htm=off in this case. This patch does that as well.
> > > 
> > > Signed-off-by: Greg Kurz <address@hidden>
> > > ---
> > > v2: - have the testcases to pass cap-htm=off instead of violating the
> > >       capabilities logic.
> > > 
> > > Upstream doesn't need anything like that since newer pseries machine types
> > > start with HTM disabled by default. This is really a oneshot fix for 
> > > 2.11.2,
> > > and I've tried to make it as small as possible.
> > > 
> > > This is a full replacement of the previous version. It is based on Mike's
> > > staging tree for 2.11:  
> > 
> > Thanks for fixing this up
> > 
> > Reviewed-by: David Gibson <address@hidden>
> > 
> > Btw, 2.11.z should probably have the 2.12 machine type removed
> > entirely, as well as (obviously) not being the default.  Not within
> > scope for this patch, though.
> > 
> 
> Well... it is indeed weird but I don't think it hurts. Also, people
> may have started using pseries-2.12 with QEMU 2.11.1 (I seem to
> remember Mike told me about something like this the other day)...

I came back to thinking about this and realized this is a terrible
idea.  The problem is that because of the way we define the latest
machine type, then backwards compat props for the earlier ones, it's
very likely that the "pseries-2.12" in 2.11 won't be the same as
pseries-2.12 in 2.12, simply because 2.11 won't have the necessary
features to implement pseries-2.12 as in 2.12.


-- 
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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