linux-kernel - Re: [PATCH] vfio-pci: PCI hot reset interface

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Message-ID: <1376956328.25016.105.camel@pasglop>
Date:	Tue, 20 Aug 2013 09:52:08 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] vfio-pci: PCI hot reset interface

On Mon, 2013-08-19 at 16:59 -0600, Alex Williamson wrote:
> On Tue, 2013-08-20 at 08:42 +1000, Benjamin Herrenschmidt wrote:
> > On Mon, 2013-08-19 at 14:02 -0600, Bjorn Helgaas wrote:
> > > I guess.  And supply the pci_slot rather than the pci_dev?  I'm a
> > > little bit worried because the idea of a "slot" is not well-defined in
> > > the spec, and we have sort of an ad hoc method of discovering and
> > > managing them, e.g., acpiphp and pciehp might discover the same slot.
> > > But I guess that's no reason to bury generic code in vfio.
> > 
> > And I don't have pci_slot's at all yet on powerpc "powernv" (the host
> > platform for KVM) since at this stage we don't support physical hotplug
> > on the target machines...
> > 
> > Alex, why specifically looking for "slots" here ? I don't quite
> > understand. It makes sense to be able to reset individual devices
> > whether they are on the otherboard, behind extension chassis or directly
> > on slots...
> 
> a) resetting a slot may have a smaller footprint than resetting a bus,

*May* ... at least on PCIe there is no difference. I suppose PCI pre-E
slots might have individual reset controls.... though the way to get
them is fairly platform specific.

> b) hotplug controllers sometimes need to be involved in a bus reset.
> For b) I have a specific example where my Lenovo S20 workstation has an
> onboard tg3 NIC attached to a root port supporting pciehp (go figure
> since the tg3 is soldered onto the motherboard) and doing a secondary
> bus reset at the root port triggers a presence detection change and
> therefore tries to do a surprise removal.  By doing a "slot" reset, I
> have the hotplug controller code manage the bus reset by disabling
> presence detection around the bus reset.  If you don't have slots and
> you don't need anything special around a secondary bus reset, you're
> fine.  It's just an opportunity to provide a hook for the hotplug
> controller to participate.  Thanks,

Yuck, junk HW again ... oh well, I suppose that's never going to end...

As long as the code works without the slots I'm fine :-) As I mentioned,
we might have to do a whole different infrastructure for EEH anyway
(which sucks but we have little choice in the matter).

Cheers,
Ben.

> Alex
> 
> 
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