Re: [Xen-devel] [PATCH v5 9/9] xen/pciback: Implement PCI reset slot or bus with 'do_flr' SysFS attribute

From: Sander Eikelenboom
Date: Thu Dec 04 2014 - 11:26:11 EST

Thursday, December 4, 2014, 4:39:06 PM, you wrote:

> On Thu, 2014-12-04 at 15:50 +0100, Sander Eikelenboom wrote:
>> Thursday, December 4, 2014, 3:31:11 PM, you 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@xxxxxxxxxx> 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.
>> Depends on what you call an "incorrect fix" .. it fixes a quirk ..
>> you can say that's incorrect, but then you would have to remove 50% of
>> the kernel and Xen code as well.
>> (i do in general agree it's better to strive for a generic solution though,
>> that's exactly why i brought up that that function doesn't seem to work perfect
>> for virtualization purposes)
>> > 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.
>> Well perhaps Bjorn knows why the order of resets and skipping the rest as
>> implemented in "pci.c:__pci_dev_reset" was implemented in that way ?
>> Especially what is the expectation about pci_dev_specific_reset doing a proper
>> reset for say a vga-card:
>> - i know it doesn't work on a radeon card (doesn't blank screen, on next guest
>> boot reports it's already posted, powermanagement doesn't work).
>> - while with a slot/bus reset, that all just works fine, screen blanks
>> immediately and everything else also works.
>> Added Alex as well since he added this workaround for KVM/vfio-pci, perhaps he knows why
>> he introduced the workaround in vfio-pci instead of trying to fix it in core pci
>> code ?

> I don't know what workaround you're talking about. As devices are
> released from the user, vfio-pci attempts to reset them. If
> pci_reset_function() returns success we mark the device clean, otherwise
> it gets marked dirty. Each time a device is released, if there are
> dirty devices we test whether we can try a bus/slot reset to clean them.
> In the case of assigning a GPU this typically means that the GPU or
> audio function come through first, there's no reset mechanism so it gets
> marked dirty, the next device comes through and we manage to try a bus
> reset. vfio-pci does not have any device specific resets, all
> functionality is added to the PCI-core, thank-you-very-much. I even
> posted a generic PCI quirk patch recently that marks AMD VGA PM reset as
> bad so that pci_reset_function() won't claim that worked. All VGA
> access quirks are done in QEMU, the kernel doesn't have any business in
> remapping config space over MMIO regions or trapping other config space
> backdoors.

Thanks for your insightful reply!

With "workaround" I was trying to refer to "vfio_pci_try_bus_reset()" which
implements how to reset the devices, it indeed uses function you introduced in
pci core code (with a solution for locking issues Konrad also seems to have
ran into:

David seems to be arguing the whole "vfio_pci_try_bus_reset()" should be not
needed and just doing calling "pci_reset_function()" (directly or by
echo "1" > /sys/bus/pci/devices/BDF/reset shoud always magically do the
right thing.
(Which in my opinion seems the contradict with the mere existence
of "vfio_pci_try_bus_reset()" (i don't think you would have implemented it
when you would have deemed it unnecessary))

> I have never heard of problems with the dev specific reset claiming to
> work and not doing anything, there are only a few of these, it should be
> easy to debug.

> I didn't read the original patch, but the title alone of this patch is
> quite confusing. FLR is specifically a function-level-reset, so one
> would expect 'do_flr' to be function specific. The pci-sysfs 'reset'
> attribute is already function specific. If pci_reset_function() isn't
> doing the job and we need to use bus/slot reset, it's clearly not an
> FLR. Thanks,
> Alex

The name "do_flr" is coming from the Xen xl toolstack which historically has
code that tries to reset devices using a echo "BDF" > /sys/bus/pci/drivers/pciback/do_flr

But the name "do_flr" and the debug messages indeed are incorrect (it's not
doing a flr nor a D3/PM reset), confusing and should not be used.

And as you seem to have solved the locking issue for vfio-pci, it is probably
possible for xen-pciback to do the same. Instead of letting xen-pciback
work around the locking problem by deferring to the xl toolstack the resetting
logic could be kept into xen-pciback it self.
That would also mean that the sysfs attribute would be unnecessary and make
the naming issue moot.


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