Re: [PATCH] xen-pciback: allow compiling on other archs than x86

[Oleksandr Andrushchenko]

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

>
>
> Juergen