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 09:08:18 -0600
From:   Govinda Tatti <govinda.tatti@...cle.com>
To:     Jan Beulich <JBeulich@...e.com>,
        Konrad Wilk <konrad.wilk@...cle.com>
Cc:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH] Xen/pciback: Implement PCI slot or bus reset
 with 'do_flr' SysFS attribute

Jan,

Sorry for the late response. Please see below for my comments.

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.

> 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)

> And btw - I don't consider it too good an idea to post the next
> version of a patch when discussion of the prior one hasn't really
> settled yet.
Sure, In future I will not post next version of the patch until we 
complete agree
on all the review comment/fixes.Thanks.

Cheers
GOVINDA