linux-kernel - Re: Broken pci_block_user_cfg

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Date:	Thu, 25 Aug 2011 22:07:49 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jan Kiszka <jan.kiszka@....de>
Cc:	Brian King <brking@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Matthew Wilcox <matthew@....cx>
Subject: Re: Broken pci_block_user_cfg_access interface

On Thu, Aug 25, 2011 at 08:52:08PM +0200, Jan Kiszka wrote:
> On 2011-08-25 20:19, Michael S. Tsirkin wrote:
> > On Thu, Aug 25, 2011 at 03:06:01PM +0200, Jan Kiszka wrote:
> >>> I took a look at the sysfs triggered pci reset function and don't see any way
> >>> that the controlling device driver ever gets to be involved in this reset.
> >>> If code outside the ipr driver were to reset the adapter, the adapter firmware
> >>> would be left in an uninitialized state and until scsi core starts timing
> >>> out ops and driving EH, the card would be unusable. I can't imagine the
> >>> ipr driver is unique in this.
> >>
> >> Right, that's why a PCI core service is needed for coordination.
> >>
> >> Jan
> > 
> > But why do we want to trigger reset through sysfs while the
> > driver runs?
> 
> A perfectly valid race conditions are created by KVM and VFIO: shared
> IRQ handler is triggered while the user space part wants to reset the
> assigned device.

OK, that would be solved by blocking sysfs reset while
config access is blocked, right?

> I'm quite sure that this how I first caused this bug to
> show up (it triggered for an assigned device with a shared busy IRQ line
> during QEMU startup, ie. the initial guest reset).
> 
> Jan
> 


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