Re: Broken pci_block_user_cfg_access interface

From: Jan Kiszka <jan.kiszka@web.de>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Brian King <brking@linux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
	Alex Williamson <alex.williamson@redhat.com>,
	Jesse Barnes <jbarnes@virtuousgeek.org>,
	Matthew Wilcox <matthew@wil.cx>
Subject: Re: Broken pci_block_user_cfg_access interface
Date: Thu, 25 Aug 2011 20:52:08 +0200	[thread overview]
Message-ID: <4E5699D8.3070505@web.de> (raw)
In-Reply-To: <20110825181912.GB27183@redhat.com>

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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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