Re: [qemu-s390x] [Qemu-devel] [PATCH v2] s390x/pci: Set the iommu region size mpcifc request

[Pierre Morel]

On 16/01/2019 15:50, Halil Pasic wrote:
> On Wed, 16 Jan 2019 15:16:44 +0100
> Pierre Morel <address@hidden> wrote:
>
> > On 16/01/2019 13:40, Halil Pasic wrote:
> > > On Tue, 15 Jan 2019 10:35:42 -0500
> > > Collin Walling <address@hidden> wrote:
> > >
> > > > On 1/10/19 8:00 AM, Pierre Morel wrote:
> > > > > The size of the accessible iommu memory region in the guest
> > > > > is given to the IOMMU by the guest through the mpcifc request
> > > > > specifying the PCI Base Address and the PCI Address Limit.
> > > > >
> > > > > Let set the size of the IOMMU region to:
> > > > >        (PCI Address Limit) - (PCI Base Address) + 1.
> > > > >
> > > > > Signed-off-by: Pierre Morel <address@hidden>
> > > > > ---
> > > > >     hw/s390x/s390-pci-bus.c | 2 +-
> > > > >     1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
> > > > > index 69e0671..e97696a 100644
> > > > > --- a/hw/s390x/s390-pci-bus.c
> > > > > +++ b/hw/s390x/s390-pci-bus.c
> > > > > @@ -660,7 +660,7 @@ void s390_pci_iommu_enable(S390PCIIOMMU *iommu)
> > > > >         char *name = g_strdup_printf("iommu-s390-%04x", iommu->pbdev->uid);
> > > > >         memory_region_init_iommu(&iommu->iommu_mr, sizeof(iommu->iommu_mr),
> > > > >                                  TYPE_S390_IOMMU_MEMORY_REGION, 
> > > > > OBJECT(&iommu->mr),
> > > > > -                             name, iommu->pal + 1);
> > > > > +                             name, iommu->pal - iommu->pba + 1);
> > >
> > >   From the the look of this, I would say we basically used the address
> > > denoting the end of the region as the size of the region. This smells
> > > like a bug to me, but the commit message and the title ain't clear about
> > > this, and there is no fixes tag. Because of the latter I did some digging
> > > and came to commit f7c40aa "s390x/pci: fix failures of dma
> > > map/unmap" (Yi Min Zhao, 2016-06-19) which basically did the inverse of
> > > this commit!
> > >
> > > My initial motivation was to check if this is stable material. But now
> > > I'm very confused. I'm admittedly zPCI incompetent. Could some of the
> > > people that understand what is going on help me feel better about this
> > > patch?
> > >
> > > Regards,
> > > Halil
> >
> >
> > The patch you speak about corrected the problem described in its comment
> > by setting the offset address of the subregion to 0, making sure
> > VFIO_PCI works for Z but introduced a bug we did not see at that time by
> > making the subregion too large.
> >
> > This patch correct the bug, I can add a reference to this with:
> > fixing: commit f7c40aa1e7feb50bc4d4bc171fa811bdd9a93e51
>
> @Connie, will you add the Fixes tag? Do we need a cc stable (since
> broken since 2016-06-19)?
>
> @Pierre: So you say it's a bug.
> What can go wrong because of this?
> For example if we interpret pal as a size, I guess we could end up with
> the memory region not fitting the guest memory,

The memory region will be too large compared with what the guest required.

> or? I'm still pretty
> much in the dark about the implications of this bug.
>
> Regards,
> Halil


--
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany