Date:   Tue, 21 Sep 2021 09:09:12 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Cc:     "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 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.

> 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)".


Juergen

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