Date:   Tue, 15 Jun 2021 02:12:15 +0300
From:   Max Gurtovoy <mgurtovoy@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     Jason Gunthorpe <jgg@...dia.com>, <cohuck@...hat.com>,
        <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <aviadye@...dia.com>, <oren@...dia.com>, <shahafs@...dia.com>,
        <parav@...dia.com>, <artemp@...dia.com>, <kwankhede@...dia.com>,
        <ACurrid@...dia.com>, <cjia@...dia.com>, <yishaih@...dia.com>,
        <kevin.tian@...el.com>, <hch@...radead.org>, <targupta@...dia.com>,
        <shameerali.kolothum.thodi@...wei.com>, <liulongfang@...wei.com>,
        <yan.y.zhao@...el.com>
Subject: Re: [PATCH 09/11] PCI: add matching checks for driver_override
 binding


On 6/14/2021 9:42 PM, Alex Williamson wrote:
> On Sun, 13 Jun 2021 11:19:46 +0300
> Max Gurtovoy <mgurtovoy@...dia.com> wrote:
>
>> On 6/9/2021 4:27 AM, Alex Williamson wrote:
>>> On Tue, 8 Jun 2021 19:45:17 -0300
>>> Jason Gunthorpe <jgg@...dia.com> wrote:
>>>   
>>>> On Tue, Jun 08, 2021 at 03:26:43PM -0600, Alex Williamson wrote:
>>>>>> drivers that specifically opt into this feature and the driver now has
>>>>>> the opportunity to provide a proper match table that indicates what HW
>>>>>> it can properly support. vfio-pci continues to support everything.
>>>>> In doing so, this also breaks the new_id method for vfio-pci.
>>>> Does it? How? The driver_override flag is per match entry not for the
>>>> entire device so new_id added things will work the same as before as
>>>> their new match entry's flags will be zero.
>>> Hmm, that might have been a testing issue; combining driverctl with
>>> manual new_id testing might have left a driver_override in place.
>>>      
>>>>> Sorry, with so many userspace regressions, crippling the
>>>>> driver_override interface with an assumption of such a narrow focus,
>>>>> creating a vfio specific match flag, I don't see where this can go.
>>>>> Thanks,
>>>> On the other hand it overcomes all the objections from the last go
>>>> round: how userspace figures out which driver to use with
>>>> driver_override and integrating the universal driver into the scheme.
>>>>
>>>> pci_stub could be delt with by marking it for driver_override like
>>>> vfio_pci.
>>> By marking it a "vfio driver override"? :-\
>>>   
>>>> But driverctl as a general tool working with any module is not really
>>>> addressable.
>>>>
>>>> Is the only issue the blocking of the arbitary binding? That is not a
>>>> critical peice of this, IIRC
>>> We can't break userspace, which means new_id and driver_override need
>>> to work as they do now.  There are scads of driver binding scripts in
>>> the wild, for vfio-pci and other drivers.  We can't assume such a
>>> narrow scope.  Thanks,
>> what about the following code ?
>>
>> @@ -152,12 +152,28 @@ static const struct pci_device_id
>> *pci_match_device(struct pci_driver *drv,
>>           }
>>           spin_unlock(&drv->dynids.lock);
>>
>> -       if (!found_id)
>> -               found_id = pci_match_id(drv->id_table, dev);
>> +       if (found_id)
>> +               return found_id;
> a) A dynamic ID match always works regardless of driver override...
>
>> -       /* driver_override will always match, send a dummy id */
>> -       if (!found_id && dev->driver_override)
>> +       found_id = pci_match_id(drv->id_table, dev);
>> +       if (found_id) {
>> +               /*
>> +                * if we found id in the static table, we must fulfill the
>> +                * matching flags (i.e. if PCI_ID_F_DRIVER_OVERRIDE flag is
>> +                * set, driver_override should be provided).
>> +                */
>> +               bool is_driver_override =
>> +                       (found_id->flags & PCI_ID_F_DRIVER_OVERRIDE) != 0;
>> +               if ((is_driver_override && !dev->driver_override) ||
> b) A static ID match fails if the driver provides an override flag and
> the device does not have an override set, or...
>
>> +                   (dev->driver_override && !is_driver_override))
> c) The device has an override set and the driver does not support the
> override flag.
>
>> +                       return NULL;
>> +       } else if (dev->driver_override) {
>> +               /*
>> +                * if we didn't find suitable id in the static table,
>> +                * driver_override will still , send a dummy id
>> +                */
>>                   found_id = &pci_device_id_any;
>> +       }
>>
>>           return found_id;
>>    }
>>
>>
>> dynamic ids (new_id) works as before.
>>
>> Old driver_override works as before.
> This is deceptively complicated, but no, I don't believe it does.  By
> my understanding of c) an "old" driver can no longer use
> driver_override for binding a known device.  It seems that if we have a
> static ID match, then we cannot have a driver_override set for the
> device in such a case.  This is a userspace regression.

If I'll remove condition c) everyone will be happy ?

I really would like to end this ongoing discussion and finally have a 
clear idea of what we want.

By clear I mean C code.

If we'll continue raising ideas we'll never reach our goal. And my goal 
is the next merge window.

>
>> For "new" driver_override we must fulfill the new rules.
> For override'able drivers, the static table is almost useless other
> than using it for modules.alias support and potentially to provide
> driver_data.  As above, I find this all pretty confusing and I'd advise
> trying to write a concise set of rules outlining the behavior of
> driver_override vs dynamic IDs vs static IDs vs "override'able" driver
> flags.  I tried, I can't, it's convoluted and full of exceptions.
> Thanks,
>
> Alex
>

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