Re: [Xen-devel] libxl__device_pci_reset() questions

[Sander Eikelenboom <linux@xxxxxxxxxxxxxx>]

Thursday, February 19, 2015, 3:30:52 PM, you wrote:

> On Thu, 2015-02-19 at 13:59 +0000, Jan Beulich wrote:
>> All,
>> 
>> in the context of someone seeing "The kernel doesn't support reset
>> from sysfs for PCI device", is my understanding correct that the lack
>> of error checking in any caller (perhaps intentional) means that any
>> of the errors logged from this function are really just warnings, i.e.
>> don't prevent the assignment from taking place?

> It was a long while ago, but I believe that was the intention, yes.

>> Furthermore I'm puzzled by the function first thing trying to access
>> a "do_flr" file supposedly made available by the pciback driver, yet
>> I can't see either the upstream or the old 2.6.18 driver surfacing
>> such a file. What am I missing here?

> I'm not sure, on the basis of
> http://lists.xen.org/archives/html/xen-devel/2014-06/msg03105.html and 
> http://lists.xen.org/archives/html/xen-devel/2014-07/msg01108.html I've
> added Konrad to the CC.

You seemed to have missed the actual CC, added now and also David who nacked 
these patches.

The fundamental issue seems to be:
A) Does this even have to be sysfs triggered functionality. The most prominent 
   reason for this construct with pciback deferring it to the toolstack which 
in 
   turn invokes a sysfs to pciback again were issues around locking.
   However vfio/virtio also has the bus/slot reset logic and seems to be able to
   work around said locking issues.

B) If it *has* to be done with a sysfs entry, the naming of the sysfs entry 
   "do_flr" is misleading since it doesn't do an FLR, but hooks up bus/slot 
   reset.

That said, i'm running for quite some time now with the mentioned do_flr patches
from Konrad applied, these fix issues with pci devices not being properly reset 
when stopping and starting VM's with pci devices passed through. Most 
prominently VGA cards (without it they won't work any more after a VM shutdown
and you would have to restart the host (you also see the screen isn't reset to 
all black after the VM shuts down.)

So the functionality is valuable, but it would be nice if the implementation 
could be contained within pciback (when a device is being mappend/ unmapped 
from a guest) and the sysfs attribute could be dropped (solves the naming issue)

--
Sander

> Ian.





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