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

From: Guenter Roeck
Date: Thu Aug 15 2013 - 18:18:54 EST

On 08/15/2013 02:49 PM, Peter Maydell wrote:
On 15 August 2013 21:50, Guenter Roeck <linux@xxxxxxxxxxxx> 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:
So it's better for the kernel to make sure it gets the
behaviour it wants rather than getting unpleasant surprises

But doesn't that mean that there is _currently_ no problem ? If so,
we can introduce the additional code when the problem really shows up.
Being Preemptive is good, but if it is not really needed today
I would rather have today's problems resolved and bother about tomorrow's
when they show up.


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