linux-kernel - Re: [Xen-devel] [PATCH v5 9/9] xen/pciback: Implement PCI reset slot or bus with 'do

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Message-ID: <20141204190514.GB4545@laptop.dumpdata.com>
Date:	Thu, 4 Dec 2014 14:05:14 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>, alex.williamson@...hat.com
Cc:	Sander Eikelenboom <linux@...elenboom.it>, bhelgaas@...gle.com,
	linux-pci@...r.kernel.org,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [Xen-devel] [PATCH v5 9/9] xen/pciback: Implement PCI reset slot
 or bus with 'do_flr' SysFS attribute

On Thu, Dec 04, 2014 at 02:31:11PM +0000, David Vrabel wrote:
> On 04/12/14 14:09, Sander Eikelenboom wrote:
> > 
> > Thursday, December 4, 2014, 2:43:06 PM, you wrote:
> > 
> >> On 04/12/14 13:10, Sander Eikelenboom wrote:
> >>>
> >>> Thursday, December 4, 2014, 1:24:47 PM, you wrote:
> >>>
> >>>> On 04/12/14 12:06, Konrad Rzeszutek Wilk wrote:
> >>>>>
> >>>>> On Dec 4, 2014 6:30 AM, David Vrabel <david.vrabel@...rix.com> wrote:
> >>>>>>
> >>>>>> On 03/12/14 21:40, Konrad Rzeszutek Wilk wrote: 
> >>>>>>>
> >>>>>>> Instead of doing all this complex dance, we depend on the toolstack 
> >>>>>>> doing the right thing. As such implement the 'do_flr' SysFS attribute 
> >>>>>>> which 'xl' uses when a device is detached or attached from/to a guest. 
> >>>>>>> It bypasses the need to worry about the PCI lock. 
> >>>>>>
> >>>>>> No.  Get pciback to add its own "reset" sysfs file (as I have repeatedly 
> >>>>>> proposed). 
> >>>>>>
> >>>>>
> >>>>> Which does not work as the kobj will complain (as there is already an 'reset' associated with the PCI device).
> >>>
> >>>> It is only needed if the core won't provide one.
> >>>
> >>>> +static int pcistub_try_create_reset_file(struct pci_dev *pci)
> >>>> +{
> >>>> +       struct xen_pcibk_dev_data *dev_data = pci_get_drvdata(pci);
> >>>> +       struct device *dev = &pci->dev;
> >>>> +       int ret;
> >>>> +
> >>>> +       /* Already have a per-function reset? */
> >>>> +       if (pci_probe_reset_function(pci) == 0)
> >>>> +               return 0;
> >>>> +
> >>>> +       ret = device_create_file(dev, &dev_attr_reset);
> >>>> +       if (ret < 0)
> >>>> +               return ret;
> >>> +       dev_data->>created_reset_file = true;
> >>>> +       return 0;
> >>>> +}
> >>>
> >>> Wouldn't the "core-reset-sysfs-file" be still wired to the end up calling 
> >>> "pci.c:__pci_dev_reset" ?
> >>>
> >>> The problem with that function is that from my testing it seems that the 
> >>> first option "pci_dev_specific_reset" always seems to return succes, so all the
> >>> other options are skipped (flr, pm, slot, bus). However the device it self is 
> >>> not properly reset enough (perhaps the pci_dev_specific_reset is good enough for 
> >>> none virtualization purposes and it's probably the least intrusive. For 
> >>> virtualization however it would be nice to be sure it resets properly, or have a 
> >>> way to force a specific reset routine.)
> > 
> >> Then you need work with the maintainer for those specific devices or
> >> drivers to fix their specific reset function.
> > 
> >> I'm not adding stuff to pciback to workaround broken quirks.
> > 
> > OK that's a pretty clear message there, so if one wants to use pci and vga
> > passthrough one should better use KVM and vfio-pci.
> 
> Have you (or anyone else) ever raised the problem with the broken reset
> quirk for certain devices with the relevant maintainer?
> 
> > vfio-pci has:
> > - logic to do the try-slot-bus-reset logic
> 
> Just because vfio-pci fixed it incorrectly doesn't mean pciback has to
> as well.
> 
> It makes no sense for both pciback and vfio-pci to workaround problems
> with pci_function_reset() in different ways -- it should be fixed in the
> core PCI code so both can benefit and make use of the same code.

We seem to be going in circles.

This thread: http://patchwork.ozlabs.org/patch/368668/

has an interesting discussion that pretty much touches all of this
and I believe it ends with David agreeing with Alex that an
bus-reset is a perfect use-case.

I believe the contention was on how to expose this interface
to the user-space. David's was thinking to use 'reset' while
I used 'do_flr' (which is a misleading name but hte toolstack
already does it). Perhaps we should just have (as David suggested)
an 'bus_reset' on the devices.

I am going to go on a limb and presume that David was thinking
that this 'bus_reset' would be exposed in the generic PCI land?

> 
> David
> 
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