Re: [RFC 0/2] Identify aliased maps in vdpa SVQ iova_tree

[Eugenio Perez Martin <eperezma@redhat.com>]

On Fri, Apr 12, 2024 at 8:47 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Wed, Apr 10, 2024 at 6:03 PM Eugenio Pérez <eperezma@redhat.com> wrote:
> >
> > The guest may have overlapped memory regions, where different GPA leads
> > to the same HVA.  This causes a problem when overlapped regions
> > (different GPA but same translated HVA) exists in the tree, as looking
> > them by HVA will return them twice.
>
> I think I don't understand if there's any side effect for shadow virtqueue?
>

My bad, I totally forgot to put a reference to where this comes from.

Si-Wei found that during initialization this sequences of maps /
unmaps happens [1]:

HVA                    GPA                IOVA
-------------------------------------------------------------------------------------------------------------------------
Map
[0x7f7903e00000, 0x7f7983e00000)    [0x0, 0x80000000) [0x1000, 0x80000000)
[0x7f7983e00000, 0x7f9903e00000)    [0x100000000, 0x2080000000)
[0x80001000, 0x2000001000)
[0x7f7903ea0000, 0x7f7903ec0000)    [0xfeda0000, 0xfedc0000)
[0x2000001000, 0x2000021000)

Unmap
[0x7f7903ea0000, 0x7f7903ec0000)    [0xfeda0000, 0xfedc0000) [0x1000,
0x20000) ???

The third HVA range is contained in the first one, but exposed under a
different GVA (aliased). This is not "flattened" by QEMU, as GPA does
not overlap, only HVA.

At the third chunk unmap, the current algorithm finds the first chunk,
not the second one. This series is the way to tell the difference at
unmap time.

[1] https://lists.nongnu.org/archive/html/qemu-devel/2024-04/msg00079.html

Thanks!

> Thanks
>
> >
> > To solve this, track GPA in the DMA entry that acs as unique identifiers
> > to the maps.  When the map needs to be removed, iova tree is able to
> > find the right one.
> >
> > Users that does not go to this extra layer of indirection can use the
> > iova tree as usual, with id = 0.
> >
> > This was found by Si-Wei Liu <si-wei.liu@oracle.com>, but I'm having a hard
> > time to reproduce the issue.  This has been tested only without overlapping
> > maps.  If it works with overlapping maps, it will be intergrated in the main
> > series.
> >
> > Comments are welcome.  Thanks!
> >
> > Eugenio Pérez (2):
> >   iova_tree: add an id member to DMAMap
> >   vdpa: identify aliased maps in iova_tree
> >
> >  hw/virtio/vhost-vdpa.c   | 2 ++
> >  include/qemu/iova-tree.h | 5 +++--
> >  util/iova-tree.c         | 3 ++-
> >  3 files changed, 7 insertions(+), 3 deletions(-)
> >
> > --
> > 2.44.0
> >
>