linux-kernel - Re: Broken pci_block_user_cfg

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Message-ID: <4E5699D8.3070505@web.de>
Date:	Thu, 25 Aug 2011 20:52:08 +0200
From:	Jan Kiszka <jan.kiszka@....de>
To:	"Michael S. Tsirkin" <mst@...hat.com>
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 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. 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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