Re: [Xen-devel] [PATCH v2 0/4] Fix device listener interface for PCI to PCI bridges

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Tue, Jun 09, 2015 at 09:18:49AM +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: Michael S. Tsirkin [mailto:mst@xxxxxxxxxx]
> > Sent: 09 June 2015 10:13
> > To: Don Slutz
> > Cc: qemu-devel@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxx; Paul Durrant;
> > Stefano Stabellini
> > Subject: Re: [PATCH v2 0/4] Fix device listener interface for PCI to PCI 
> > bridges
> > 
> > On Mon, Jun 08, 2015 at 05:18:48PM -0400, Don Slutz wrote:
> > > changes v1 to v2:
> > >   Split v1 patch into 3.
> > >
> > >   Added a BUG FIX patch (#1: "exec: Do not use MemoryRegion after
> > >   free").
> > >
> > >     Technically this bug fix should be a separate patch, however this
> > >     issue only seems to reproduce when this patch set is applied.
> > >
> > >
> > >   Michael S. Tsirkin:
> > >     "You need some other API that makes sense, probably PCI specific."
> > >       This is basically patch #2: "Extend device listener interface..."
> > >
> > >     "This is relying on undocumented assumptions and how specific
> > >     firmware works. There's nothing special about bus number 255,
> > >     and 0 is not very special either (except it happens to be the
> > >     reset value)."
> > >       Dropped all checking of 0 and 255.
> > >
> > >
> > > Open question by Michael S. Tsirkin:
> > >
> > > >>>> On Thu, May 28, 2015 at 07:25:50AM -0400, Don Slutz wrote:
> > > ...
> > > >>>> It is not clear to me that the complexity of tracking bus
> > > >>>> visibility make sense.  Clearly you do.
> > > >>> Well what was the point of the change?
> > > >
> > > > To get config cycles that are valid that you do not get today.
> > > >
> > > >>> If you don't care that we get irrelevant config cycles why not
> > > >>> just send them all to QEMU?
> > > >>>
> > > >
> > > > That would need to be answered by Paul Durrant and/or other people
> > (see
> > > > below)
> > > >
> > >
> > > We could broadcast config space ioreqs to all emulators, the problem
> > > is how do we know (in the case of a read) which emulator is actually
> > > the one supplying the data? At some level Xen needs to know who is
> > > implementing what.
> > >
> > >   Paul Durrant
> > 
> > Can irrelevant emulators all respond with some kind of nack?
> > Xen would pick the one that did respond correctly.
> > 
> 
> I guess that's possible if we add an extra bit to the ioreq_t, but QEMU would 
> still need to know when to nack and when to ack.

It's simple: ack if we find a device handling the specific BDF.
The result would at least be contained.
OTOH detecting master aborts in core is useful since it would
let us implement error reporting correctly.


> It's still much simpler if qemu updates Xen with exact set of (S)BDFs it's 
> handling.
> 
>   Paul


I suspect this calls for a bigger change, you need to re-scan
all of the tree to detect visible devices.
Maybe just write some xen-specific code to do this on each
config access.


> > > So this patch set just adjusts Xen to correctly know the current set
> > > of PCI devices and their bus, device, and function.
> > >
> > > It does not attempt to calculate the visibility of the PCI devices.
> > 
> > So non-visible devices will appear with invalid bus number values,
> > conflicting with the visible devices.
> > Seems wrong.
> > 
> > 
> > >
> > > v1:
> > >
> > > Note: Right now hvmloader in Xen does not handle PCI to PCI bridges
> > > and so SeaBIOS does not have access to PCI device(s) on secondary
> > > buses.  How ever domU can setup the secondary bus(es) and this patch
> > > will restore access to these PCI devices.
> > >
> > >
> > > Don Slutz (4):
> > >   exec: Do not use MemoryRegion after free
> > >   Extend device listener interface for PCI to PCI bridges
> > >   xen: Add usage of device listener interface for PCI to PCI bridges
> > >   xen: Fix map/unmap of pcidev to ioreq server
> > >
> > >  exec.c                      |  8 ++++--
> > >  hw/core/qdev.c              |  7 ++++++
> > >  hw/pci/pci_bridge.c         | 18 +++++++++++++
> > >  include/hw/qdev-core.h      |  3 +++
> > >  include/hw/xen/xen_common.h | 61
> > ++++++++++++++++++++++++++++++++++-----------
> > >  trace-events                |  6 +++--
> > >  xen-hvm.c                   | 20 +++++++++++++--
> > >  7 files changed, 102 insertions(+), 21 deletions(-)
> > >
> > > --
> > > 1.8.4

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