Date:   Tue, 21 Sep 2021 13:44:06 -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 Tue, 21 Sep 2021, Oleksandr Andrushchenko wrote:
> On 21.09.21 10:09, Juergen Gross wrote:
> > On 21.09.21 09:00, Oleksandr Andrushchenko wrote:
> >>
> >> On 21.09.21 09:49, Juergen Gross wrote:
> >>> On 21.09.21 08:38, Oleksandr Andrushchenko wrote:
> >>>>
> >>>> On 21.09.21 09:07, Juergen Gross wrote:
> >>>>> 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 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.
> >>>>>>
> >>>>>> As the first stage before the driver is split or ifdef's used - can we take the patch
> >>>>>> 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.
> >>>> Of course
> >>>>>
> >>>>>>
> >>>>>>>>
> >>>>>>>> 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?
> >>>>>>>
> >>>>>>> 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 *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);
> >>>>>>>
> >>>>>>> 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 xen_pcibk_backend;
> >>>>>>>>        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.
> >>>>
> >>>> Hm, pciback is now compiled when CONFIG_XEN_PCIDEV_BACKEND is enabled
> >>>> and we want to skip some parts of its code when CONFIG_XEN_PCI_STUB is set.
> >>>> So, I imagine that for x86 we just enable CONFIG_XEN_PCIDEV_BACKEND and the
> >>>> driver compiles in its current state. For Arm we enable both CONFIG_XEN_PCIDEV_BACKEND
> >>>> and CONFIG_XEN_PCI_STUB, so part of the driver is not compiled.
> >>>
> >>> No, I'd rather switch to compiling xen-pciback when CONFIG_XEN_PCI_STUB
> >>> is set and compile only parts of it when CONFIG_XEN_PCIDEV_BACKEND is
> >>> not set (this will be the case on Arm).
> >>
> >> But this will require that the existing kernel configurations out there have to additionally define CONFIG_XEN_PCI_STUB to get what they had before with simply enabling CONFIG_XEN_PCIDEV_BACKEND. My point was that it is probably desirable not to break
> >> the things while doing the split/re-work.
> >
> > By letting XEN_PCIDEV_BACKEND select XEN_PCI_STUB this won't happen.
> Indeed
> >
> >> I also thought that "compile only parts of it when CONFIG_XEN_PCIDEV_BACKEND is not set"
> >> may have more code gated rather than with gating unwanted code with CONFIG_XEN_PCI_STUB.
> >> I am not quite sure about this though.
> >
> > This would be a very weird semantics of XEN_PCI_STUB, as the stub part
> > is needed on X86 and on Arm.
> >
> > Gating could even be done with Stefano's patch just by replacing his
> > "!xen_pv_domain()" tests with "!IS_ENABLED(CONFIG_XEN_PCIDEV_BACKEND)".
> 
> Makes sense.
> 
> Another question if we do not want the code to be compiled or not executed?
> 
> If the later then we can define something like:
> 
> bool need_pv_part(void)
> 
> {
> 
>      return IS_ENABLED(CONFIG_XEN_PCIDEV_BACKEND);
> 
> }
> 
> and then just use need_pv_part() for the checks where it is needed.
> 
> This allows avoiding multiple ifdef's through the code

This is even better.

For my clarity, Oleksandr, are you OK with adding a few need_pv_part()
checks through the code as part of this series so that the PV PCI
backend is not initialized?

I don't have a good test environment ready for this, so I cannot really
volunteer myself.

I would prefer if we made this change as part of this series so that the
PV PCI backend features doesn't get enabled on ARM, not even temporarily.

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