Date:   Tue, 21 Sep 2021 07:00:39 +0000
From:   Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>
To:     Juergen Gross <jgross@...e.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 09:49, 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.

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 is another step in the right direction preparing the split.
>
> But as said before, this is not a requirement by me to take your patch.
Thank you
>
>
> Juergen

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