Re: [PATCH] x86/IRQ: fix valid-old-vector checks in __assign_irq_vector()

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 27/09/12 16:04, Jan Beulich wrote:
>>>> On 27.09.12 at 16:57, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>> On 27/09/12 15:50, Jan Beulich wrote:
>>> There are two greater-than-zero checks for the old vector retrieved,
>>> which don't work when a negative value got stashed into the respective
>>> arch_irq_desc field. The effect of this was that for interrupts that
>>> are intended to get their affinity adjusted the first time before the
>>> first interrupt occurs, the affinity change would fail, because the
>>> original vector assignment would have caused the move_in_progress flag
>>> to get set (which causes subsequent re-assignments to fail until it
>>> gets cleared, which only happens from the ->ack() actor, i.e. when an
>>> interrupt actually occurred).
>>>
>>> This addresses a problem introduced in c/s 23816:7f357e1ef60a (by
>>> changing IRQ_VECTOR_UNASSIGNED from 0 to -1).
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>> ---
>>> I have to admit that I don't understand why the value got changed in
>>> the first place: 0 is as invalid a value as -1 for a vector to be used
>>> for delivering hardware interrupts.
>> http://lists.xen.org/archives/html/xen-devel/2011-09/msg00193.html 
>>
>> It was a suggestion for consistency with using -1 elsewhere in the irq
>> code to mean unassigned.
> Not really - there George suggested to use IRQ_VECTOR_UNASSIGNED,
> but not to make that resolve to -1. My claim is that this manifest
> constant could easily resolve to zero instead.
>
> Jan

Ah - it was in the following email.

"Yes - I missed that.  However, IRQ_VECTOR_UNASSIGNED should be -1
instead of 0, as the first 32 entries of irq_vector have 0 entries which
are not unassigned."

Which was my justification of using -1 as opposed to 0.

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Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
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