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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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 bound
> >>      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 PCI
> >>      devices, so we can re-bind the devices back to their original drivers 
> >> when
> >>      guest domain shuts down)
> >>
> >> 3. Device reset
> >>
> >> We have previously discussed on xen-devel ML possible solutions to that as 
> >> from the
> >> above we see that pciback functionality is going to be only partially used 
> >> on Arm.
> >>
> >> 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 
> >> passthrough
> >> on Arm. It was enough to just re-structure the driver and have it run on 
> >> Arm to achieve
> >> 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 take.
> >
> > 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.

I am wonder if there is a simple:

if (!xen_pv_domain())
    return;

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?


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 
*dev,
                                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);
        if (pdev == NULL) {
                err = -ENOMEM;
                xenbus_dev_fatal(dev, err,
@@ -743,6 +748,9 @@ const struct xen_pcibk_backend *__read_mostly 
xen_pcibk_backend;
 
 int __init xen_pcibk_xenbus_register(void)
 {
+       if (!xen_pv_domain())
+               return 0;
+
        xen_pcibk_backend = &xen_pcibk_vpci_backend;
        if (passthrough)
                xen_pcibk_backend = &xen_pcibk_passthrough_backend;
@@ -752,5 +760,7 @@ int __init xen_pcibk_xenbus_register(void)
 
 void __exit xen_pcibk_xenbus_unregister(void)
 {
+       if (!xen_pv_domain())
+               return;
        xenbus_unregister_driver(&xen_pcibk_driver);
 }