linux-kernel - Re: [Qemu-devel] SCSI bus failures with qemu-arm in kernel 3.8+

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Date:	Thu, 15 Aug 2013 22:49:05 +0100
From:	Peter Maydell <peter.maydell@...aro.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	QEMU Developers <qemu-devel@...gnu.org>,
	Arnd Bergmann <arnd.bergmann@...aro.org>
Subject: Re: [Qemu-devel] SCSI bus failures with qemu-arm in kernel 3.8+

On 15 August 2013 21:50, Guenter Roeck <linux@...ck-us.net> wrote:
> On Thu, Aug 15, 2013 at 07:05:22PM +0100, Peter Maydell wrote:
>> It needs to go in the same patch, because a kernel with the fixed
>> irq remapping must also tell QEMU it is fixed; if you split the
>> two then at the point between the two patches the kernel is
>> broken for bisection purposes.
>>
> Thinking about it - is that really true ? My image with the
> patch applied works just fine under qemu 1.5.2, and unless
> I am missing something it won't work with qemu 1.4 anyway.
> So what exactly is broken ?

You're OK unless the kernel happens to pick the same interrupt
number to write to PCI_INTERRUPT_LINE as one of the previous
broken kernel versions did (in which case QEMU will incorrectly
assume you're a broken kernel). This can't happen with the way
the kernel is currently picking interrupt numbers (ie with a
straightforward relationship between h/w irqs and values written),
but as I understand from Arnd there is a plan to move to a
different approach ("sparse irqs") at which point this won't hold:
http://lists.gnu.org/archive/html/qemu-devel/2013-03/msg04579.html
So it's better for the kernel to make sure it gets the
behaviour it wants rather than getting unpleasant surprises
later.

-- PMM
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