linux-kernel - Re: [PATCH v3 4/7] xen/pciback: Implement PCI reset slot or bus with 'do

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Message-ID: <53BD50FD.7080309@citrix.com>
Date:	Wed, 9 Jul 2014 15:26:05 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	<konrad@...nel.org>, <xen-devel@...ts.xenproject.org>,
	<boris.ostrovsky@...cle.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 4/7] xen/pciback: Implement PCI reset slot or bus with
 'do_flr' SysFS attribute

On 09/07/14 15:12, Konrad Rzeszutek Wilk wrote:
> On Wed, Jul 09, 2014 at 01:32:10PM +0100, David Vrabel wrote:
>> On 08/07/14 19:46, Konrad Rzeszutek Wilk wrote:
>>> On Tue, Jul 08, 2014 at 07:02:51PM +0100, David Vrabel wrote:
>>>> On 08/07/14 19:58, konrad@...nel.org wrote:
>>>>> --- a/Documentation/ABI/testing/sysfs-driver-pciback
>>>>> +++ b/Documentation/ABI/testing/sysfs-driver-pciback
>>>>> @@ -82,3 +82,14 @@ Description:
>>>>>                  device is shared, enabled, or on a level interrupt line.
>>>>>                  Writing a string of DDDD:BB:DD.F will toggle the state.
>>>>>                  This is Domain:Bus:Device.Function where domain is optional.
>>>>> +
>>>>> +What:           /sys/bus/pci/drivers/pciback/do_flr
>>>>> +Date:           July 2014
>>>>> +KernelVersion:  3.16
>>>>> +Contact:        xen-devel@...ts.xenproject.org
>>>>> +Description:
>>>>> +                An option to slot or bus reset an PCI device owned by
>>>>> +                Xen PCI backend. Writing a string of DDDD:BB:DD.F will cause
>>>>> +                the driver to perform an slot or bus reset if the device
>>>>> +                supports. It also checks to make sure that all of the devices
>>>>> +                under the bridge are owned by Xen PCI backend.
>>>>
>>>> Not sure I like this new interface.  I solved this by adding a new reset
>>>> file that looked like the regular one the pci would have if it supported
>>>> FLR.  I'm fairly sure I posted a series for this.  Was there a reason
>>>> you didn't do this?
>>>
>>> It did not work.
>>>
>>> During bootup kobject would complain about a secondary 'reset' SysFS
>>> on the PCI device.
>>
>> I think this because of pciback registering a driver too early, before
>> the device is fully initialized.  You can see in the trace that it is
>> the common pci code that is trying to add the "reset" file so it must be
>> doing this /after/ pciback's probe has been called.
>>
>> I would consider:
>>
>> 1. Removing the "hide" module parameter -- it doesn't work if pciback is
>> a module anyway.
> 
> I find it incredibly useful and so do a lot of other people.

PCI passthrough must work well without hide and without pciback being
built-in (and it does with the "reset" change).

What are you using "hide" for?

>> 2. Making pciback initialize like a regular driver module (no
>> fs_initcall() shenanigans).
> 
> The point is to take the PCI device before the drivers touch it.
> 
> We want it to be in a pristine state so that the device driver
> domains can use it.

But hide only ensures this the first time the device is assigned.  Using
a function reset ensures this all the time.

>> 3. Require userspace to sort out binding the device to pciback (e.g.,
>> libxl already does the unbind if requested).
> 
> How would you do the bus/slot reset? Or are you thinking that at
> that point the 'reset' functionality would be over-written to point
> to Xen pciback and it would do the job?

I'm not sure I understand your question.  libxl already does the
function reset (by writing to "reset").

>> 4. Finally, I would consider generic driver core functionality for
>> prioritizing drivers so they get probed first.
> 
> Not sure I understand why you want the drivers to use the device
> first? The point is that we can 'hide' them from the generic
> drivers and present them to the backend domains.

The pciback driver would be prioritized, so it would be probed first.

> Regardless of these - I am curious to why you don't like do_flr
> as it is even implemented in the the toolstack (but buggy) and
> it does a good job of allowing us to do slot/bus reset?

Because there is already a documented interface for resetting devices
(the "reset" file), we don't want a second interface.

David
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