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

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Date:   Wed, 29 Nov 2017 08:35:33 -0700
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Govinda Tatti" <govinda.tatti@...cle.com>,
        "Konrad Wilk" <konrad.wilk@...cle.com>
Cc:     <xen-devel@...ts.xenproject.org>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        "Juergen Gross" <jgross@...e.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] Xen/pciback: Implement PCI slot or bus
 reset with 'do_flr' SysFS attribute

>>> On 29.11.17 at 16:08, <govinda.tatti@...cle.com> wrote:
> On 11/9/2017 2:28 AM, Jan Beulich wrote:
>>>>> On 08.11.17 at 16:44, <govinda.tatti@...cle.com> wrote:
>>> On 11/7/2017 8:40 AM, Jan Beulich wrote:
>>>>>>> On 06.11.17 at 18:48, <Govinda.Tatti@...cle.COM> wrote:
>>>>> --- a/Documentation/ABI/testing/sysfs-driver-pciback
>>>>> +++ b/Documentation/ABI/testing/sysfs-driver-pciback
>>>>> @@ -11,3 +11,15 @@ Description:
>>>>>                    #echo 00:19.0-E0:2:FF > /sys/bus/pci/drivers/pciback/quirks
>>>>>                    will allow the guest to read and write to the configuration
>>>>>                    register 0x0E.
>>>>> +
>>>>> +What:           /sys/bus/pci/drivers/pciback/do_flr
>>>>> +Date:           Nov 2017
>>>>> +KernelVersion:  4.15
>>>>> +Contact:        xen-devel@...ts.xenproject.org 
>>>>> +Description:
>>>>> +                An option to perform a slot or bus reset when a PCI device
>>>>> +		is owned by Xen PCI backend. Writing a string of DDDD:BB:DD.F
>>>>> +		will cause the pciback driver to perform a slot or bus reset
>>>>> +		if the device supports it. It also checks to make sure that
>>>>> +		all of the devices under the bridge are owned by Xen PCI
>>>>> +		backend.
>>>> Why do you name this "do_flr" when you don't even try FLR, but
>>>> go to slot or then bus reset right away.
>>> Yes, I agree but xen toolstack has already been modified to
>>> consume"do_flr" attribute. Hence, we are using the
>>> function that matches with sysfs attribute.
>> That's not a valid reason imo: Right now the driver doesn't expose
>> such an attribute, so if the tool stack was trying to use it, it would
>> not do what is intended at present anyway (i.e. the code could at
>> best be called dead).
> Sure, we can consider renaming "do_flr" attribute to "pci reset" or "bus 
> reset".
> Please let me knowyour preference.

Well, that's more a question to Konrad as the maintainer.
Personally I'd prefer just "reset", as "pci" is redundant and "bus"
doesn't cover the slot variant.

>> Furthermore, contrary to what you claim in
>> your reply to Pasi, I can't see where you try an actual FLR first -
>> you go straight to pci_probe_reset_{slot,bus}(). If you actually
>> tried FLR first, only falling back to the other methods as "emulation",
>> I could certainly agree with the file name chosen.
> Currently, multiple resets are being invoked (independently) in the context
> of "xl attach/detach/shutdown/reboot".
> 
> - pci_reset_function_locked (invoked by pcistub_put_pci_dev())
>    This function tries various PCI reset methods including FLR.
> - pcistub_reset_dev (called by toolsstack based on "do_flr" attribute)

While related in a certain way, I can't really see how this addresses
the comment.

Jan