Re: [Xen-devel] libxl__device_pci_reset() questions

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Thu, 2015-02-19 at 16:09 +0100, Sander Eikelenboom wrote:
> 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

Nope, Konrad's mailman configuration is such that the list copy of the
mail ends up with his CC stripped -- that's really a feature of mailman!

> and also David who nacked these patches.

I didn't know about this.

Oh well, I'll let other fight it out as to what the correct approach is.

> 
> 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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