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

[Guenter Roeck <linux@roeck-us.net>]

On 08/15/2013 02:49 PM, Peter Maydell wrote:
> On 15 August 2013 21:50, Guenter Roeck <linux@roeck-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.
>

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.

Guenter