Re: [Qemu-devel] [Qemu-ppc] [PATCHv2 5/5] spapr: Fix 2.7<->2.8 migration of PCI host bridge

[Greg Kurz]

On Tue, 22 Nov 2016 10:15:59 +1100
David Gibson <address@hidden> wrote:

> On Mon, Nov 21, 2016 at 05:02:20PM +0100, Greg Kurz wrote:
> > On Mon, 21 Nov 2016 13:02:53 +0100
> > Thomas Huth <address@hidden> wrote:
> >   
> > > On 21.11.2016 06:31, David Gibson wrote:  
> > > > daa2369 "spapr_pci: Add a 64-bit MMIO window" subtly broke migration
> > > > from qemu-2.7 to the current version.  It split the device's MMIO
> > > > window into two pieces for 32-bit and 64-bit MMIO.
> > > > 
> > > > The patch included backwards compatibility code to convert the old
> > > > property into the new format.  However, the property value was also
> > > > transferred in the migration stream and compared with a (probably
> > > > unwise) VMSTATE_EQUAL.  So, the "raw" value from 2.7 is compared to
> > > > the new style converted value from (pre-)2.8 giving a mismatch and
> > > > migration failure.
> > > > 
> > > > Along with the actual field that caused the breakage, there are
> > > > several other ill-advised VMSTATE_EQUAL()s.  To fix forwards
> > > > migration, we read the values in the stream into scratch variables and
> > > > ignore them, instead of comparing for equality.  To fix backwards
> > > > migration, we populate those scratch variables in pre_save() with
> > > > adjusted values to match the old behaviour.
> > > > 
> > > > To permit the eventual possibility of removing this cruft from the
> > > > stream, we only include these compatibility fields if a new
> > > > 'pre-2.8-migration' property is set.  We clear it on the pseries-2.8
> > > > machine type, which obviously can't be migrated backwards, but set it
> > > > on earlier machine type versions.
> > > > 
> > > > Signed-off-by: David Gibson <address@hidden>
> > > > ---
> > > >  hw/ppc/spapr.c              |  5 +++++
> > > >  hw/ppc/spapr_pci.c          | 33 ++++++++++++++++++++++++++++-----
> > > >  include/hw/pci-host/spapr.h |  6 ++++++
> > > >  3 files changed, 39 insertions(+), 5 deletions(-)
> > > > 
> > > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > > > index 775ad2e..c3269c7 100644
> > > > --- a/hw/ppc/spapr.c
> > > > +++ b/hw/ppc/spapr.c
> > > > @@ -2772,6 +2772,11 @@ DEFINE_SPAPR_MACHINE(2_8, "2.8", true);
> > > >          .driver = TYPE_POWERPC_CPU,                 \
> > > >          .property = "pre-2.8-migration",            \
> > > >          .value    = "on",                           \
> > > > +    },                                              \
> > > > +    {                                               \
> > > > +        .driver = TYPE_SPAPR_PCI_HOST_BRIDGE,       \
> > > > +        .property = "pre-2.8-migration",            \
> > > > +        .value    = "on",                           \
> > > >      },
> > > >  
> > > >  static void phb_placement_2_7(sPAPRMachineState *spapr, uint32_t index,
> > > > diff --git a/hw/ppc/spapr_pci.c b/hw/ppc/spapr_pci.c
> > > > index e429c94..c62c1cb 100644
> > > > --- a/hw/ppc/spapr_pci.c
> > > > +++ b/hw/ppc/spapr_pci.c
> > > > @@ -1590,6 +1590,8 @@ static Property spapr_phb_properties[] = {
> > > >      DEFINE_PROP_UINT64("pgsz", sPAPRPHBState, page_size_mask,
> > > >                         (1ULL << 12) | (1ULL << 16)),
> > > >      DEFINE_PROP_UINT32("numa_node", sPAPRPHBState, numa_node, -1),
> > > > +    DEFINE_PROP_BOOL("pre-2.8-migration", sPAPRPHBState,
> > > > +                     pre_2_8_migration, false),
> > > >      DEFINE_PROP_END_OF_LIST(),
> > > >  };
> > > >  
> > > > @@ -1636,6 +1638,20 @@ static void spapr_pci_pre_save(void *opaque)
> > > >          sphb->msi_devs[i].key = *(uint32_t *) key;
> > > >          sphb->msi_devs[i].value = *(spapr_pci_msi *) value;
> > > >      }
> > > > +
> > > > +    if (sphb->pre_2_8_migration) {
> > > > +        sphb->mig_liobn = sphb->dma_liobn[0];
> > > > +        sphb->mig_mem_win_addr = sphb->mem_win_addr;
> > > > +        sphb->mig_mem_win_size = sphb->mem_win_size;
> > > > +        sphb->mig_io_win_addr = sphb->io_win_addr;
> > > > +        sphb->mig_io_win_size = sphb->io_win_size;
> > > > +
> > > > +        if ((sphb->mem64_win_size != 0)
> > > > +            && (sphb->mem64_win_addr
> > > > +                == (sphb->mem_win_addr + sphb->mem_win_size))) {
> > > > +            sphb->mig_mem_win_size += sphb->mem64_win_size;
> > > > +        }    
> > > 
> > > Should we maybe print a warning/error message in case
> > > 
> > >  sphb->mem64_win_size != 0 &&
> > >  sphb->mem64_win_addr != sphb->mem_win_addr + sphb->mem_win_size
> > > 
> > > ... assuming that this means a configuration which can not be migrated
> > > backwards?
> > >   
> > 
> > Then shouldn't we forbid pre_2_8_migration to be set when we have a
> > non-contiguous window ?  
> 
> So, yes, we could do either of these, but really I don't think it's
> worth it.  It will only happen if you have a custom constructed PHB,
> in which case it's pretty much on you to ensure that there's something
> compatible at the far end.  Restricting it here has somewhat the same
> problem as VMSTATE_EQUAL()s did - they make assumptions about what is
> and isn't sane which could be broken by future changes (in this case
> changes in the 2.7 stable tree).  They might be unlikely to change,
> but if they do things break, and the only benefit is a marginally
> better error message in cases that won't work anyway.
> 

Makes sense indeed, hence:

Reviewed-by: Greg Kurz <address@hidden>

Cheers.

--
Greg

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