Date:   Tue, 21 Sep 2021 08:07:28 +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 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.

> 
>>>
>>> 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.

> Gating also means that we are not thinking about splitting the backend driver into
> two different ones, e.g. one for "common" code and one for PV stuff.
> Otherwise this ifdefery won't be needed.

I just wanted to avoid the xen_pv_domain() tests creeping in, as
they are wrong IMO.


Juergen

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