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Message-ID: <alpine.DEB.2.21.2109201444040.17979@sstabellini-ThinkPad-T480s>
Date: Mon, 20 Sep 2021 16:16:06 -0700 (PDT)
From: Stefano Stabellini <sstabellini@...nel.org>
To: Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>
cc: Juergen Gross <jgross@...e.com>,
Stefano Stabellini <sstabellini@...nel.org>,
"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
"julien@....org" <julien@....org>,
"jbeulich@...e.com" <jbeulich@...e.com>,
Anastasiia Lukianenko <Anastasiia_Lukianenko@...m.com>,
Oleksandr Andrushchenko <andr2000@...il.com>
Subject: Re: [PATCH] xen-pciback: allow compiling on other archs than x86
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);
}
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