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Re: [PATCH] xen-pciback: allow compiling on other archs than x86

On 21.09.21 07:51, Oleksandr Andrushchenko wrote:

On 21.09.21 08:20, Juergen Gross wrote:
On 21.09.21 01:16, Stefano Stabellini wrote:
On Mon, 20 Sep 2021, Oleksandr Andrushchenko wrote:
On 20.09.21 14:30, Juergen Gross wrote:
On 20.09.21 07:23, Oleksandr Andrushchenko wrote:
Hello, Stefano!

On 18.09.21 00:45, Stefano Stabellini wrote:
Hi Oleksandr,

Why do you want to enable pciback on ARM? Is it only to "disable" a PCI
device in Dom0 so that it can be safely assigned to a DomU?
Not only that

I am asking because actually I don't think we want to enable the PV PCI
backend feature of pciback on ARM, right? That would clash with the PCI
assignment work you have been doing in Xen. They couldn't both work at
the same time.
Correct, it is not used

If we only need pciback to "park" a device in Dom0, wouldn't it be
possible and better to use pci-stub instead?

Not only that, so pci-stub is not enough

The functionality which is implemented by the pciback and the toolstack
and which is relevant/missing/needed for ARM:

1. pciback is used as a database for assignable PCI devices, e.g. xl
       pci-assignable-{add|remove|list} manipulates that list. So, whenever the
       toolstack needs to know which PCI devices can be passed through it reads
       that from the relevant sysfs entries of the pciback.

2. pciback is used to hold the unbound PCI devices, e.g. when passing through
       a PCI device it needs to be unbound from the relevant device driver and 
       to pciback (strictly speaking it is not required that the device is 
bound to
       pciback, but pciback is again used as a database of the passed through 
       devices, so we can re-bind the devices back to their original drivers 
       guest domain shuts down)

3. Device reset

We have previously discussed on xen-devel ML possible solutions to that as from 
above we see that pciback functionality is going to be only partially used on 

Please see [1] and [2]:

1. It is not acceptable to manage the assignable list in Xen itself

2. pciback can be split into two parts: PCI assignable/bind/reset handling and
the rest like vPCI etc.

3. pcifront is not used on Arm

It is neither in x86 PVH/HVM guests.
Didn't know that, thank you for pointing

So, limited use of the pciback is one of the bricks used to enable PCI 
on Arm. It was enough to just re-structure the driver and have it run on Arm to 
all the goals above.

If we still think it is desirable to break the pciback driver into "common" and 
"pcifront specific"
parts then it can be done, yet the patch is going to be the very first brick in 
that building.

Doing this split should be done, as the pcifront specific part could be
omitted on x86, too, in case no PV guests using PCI passthrough have to
be supported.
Agree, that the final solution should have the driver split

So, I think this patch is still going to be needed besides which direction we 

Some kind of this patch, yes. It might look different in case the split
is done first.

I don't mind doing it in either sequence.

With this patch we have Arm on the same page as the above mentioned x86 guests,

e.g. the driver has unused code, but yet allows Arm to function now.

At this stage of PCI passthrough on Arm it is yet enough. Long term, when

the driver gets split, Arm will benefit from that split too, but unfortunately 
I do not

have enough bandwidth for that piece of work at the moment.

That's fair and I don't want to scope-creep this simple patch asking for
an enormous rework. At the same time I don't think we should enable the
whole of pciback on ARM because it would be erroneous and confusing.

As the first stage before the driver is split or ifdef's used - can we take the 
as is now? In either way we chose this needs to be done, e.g. enable compiling
for other architectures and common code move.

Fine with me in principle. I need to take a more thorough look
at the patch, though.

I am wonder if there is a simple:

if (!xen_pv_domain())

That we could add in a couple of places in pciback to stop it from
initializing the parts we don't care about. Something along these lines
(untested and probably incomplete).

What do you guys think?

Uh no, not in this way, please. This will kill pci passthrough on x86
with dom0 running as PVH. I don't think this is working right now, but
adding more code making it even harder to work should be avoided.

diff --git a/drivers/xen/xen-pciback/xenbus.c b/drivers/xen/xen-pciback/xenbus.c
index da34ce85dc88..991ba0a9b359 100644
--- a/drivers/xen/xen-pciback/xenbus.c
+++ b/drivers/xen/xen-pciback/xenbus.c
@@ -15,6 +15,7 @@
   #include <xen/xenbus.h>
   #include <xen/events.h>
   #include <xen/pci.h>
+#include <xen/xen.h>
   #include "pciback.h"
     #define INVALID_EVTCHN_IRQ  (-1)
@@ -685,8 +686,12 @@ static int xen_pcibk_xenbus_probe(struct xenbus_device 
                   const struct xenbus_device_id *id)
       int err = 0;
-    struct xen_pcibk_device *pdev = alloc_pdev(dev);
+    struct xen_pcibk_device *pdev;
+    if (!xen_pv_domain())
+        return 0;
   +    pdev = alloc_pdev(dev);

This hunk isn't needed, as with bailing out of xen_pcibk_xenbus_register
early will result in xen_pcibk_xenbus_probe never being called.

       if (pdev == NULL) {
           err = -ENOMEM;
           xenbus_dev_fatal(dev, err,
@@ -743,6 +748,9 @@ const struct xen_pcibk_backend *__read_mostly 
     int __init xen_pcibk_xenbus_register(void)
+    if (!xen_pv_domain())
+        return 0;

Use #ifdef CONFIG_X86 instead.

The title of this patch says that we want to allow this driver for other archs
and now we want to introduce "#ifdef CONFIG_X86" which doesn't sound
right with that respect. Instead, we may want having something like a
dedicated gate for this, e.g. "#ifdef CONFIG_XEN_PCIDEV_BACKEND_SUPP_PV"
or something which is architecture agnostic.

Something like that, yes. But I'd rather use CONFIG_XEN_PCIDEV_BACKEND
acting as this gate and introduce CONFIG_XEN_PCI_STUB for the stub
functionality needed on Arm. XEN_PCIDEV_BACKEND would depend on X86 and
select XEN_PCI_STUB, while on Arm XEN_PCI_STUB could be configured if
wanted. The splitting of the driver can still be done later.

Gating also means that we are not thinking about splitting the backend driver 
two different ones, e.g. one for "common" code and one for PV stuff.
Otherwise this ifdefery won't be needed.

I just wanted to avoid the xen_pv_domain() tests creeping in, as
they are wrong IMO.


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