Should irq_chip->mask disable percpu interrupts to all cpus, or justto this cpu?

[Jeremy Fitzhardinge]

Hi,

I'm reworking Xen's interrupt handling to isolate it a bit from the
workings of the apic-based code, as Eric suggested a while back.

As I've mentioned before, Xen represents interrupts as event channels. 
There are two major classes of event channels: per-cpu and, erm, not
percpu.  Per-cpu event channels are for things like timers and IPI
function calls which are inherently per-cpu; it's meaningless to
consider, for example, migrating them from cpu to cpu.  I guess they're
analogous to the the local apic vectors.

(Non-percpu event channels can be bound to a particular cpu, and rebound
at will; I'm not worried about them here.)

Previously I allocated an irq per percpu event channel per cpu.  This
was pretty wasteful, since I need about 5-6 of them per cpu, so the
number of interrupts increases quite quickly as cpus does.  There's no
deep problem with that, but it gets fairly ugly in /proc/interrupts, and
there's some tricky corners to manage in suspend/resume.

This time around I'm allocating a single irq for each percpu interrupt
source (so one for timers, one for IPI, etc), and mapping each per-cpu
event channel to each.  But I'm wondering what the correct behaviour of
irq_chip->mask/unmask should be in this case.  Each event channel is
individually maskable, so when ->mask gets called, I can either mask all
the event channels associated with that irq, or just the one for this
cpu.  The latter makes most sense for me, but I don't quite understand
the irq infrastructure enough to know if it will have bad effects globally.

When I request the irq, I pass IRQF_PERCPU in the flags, but aside from
preventing migration, this only seems to have an effect on __do_IRQ(),
which looks like a legacy path anyway.   It seems to me that by setting
it that I'm giving the interrupt subsystem fair warning that ->mask() is
only going to disable the interrupt on this cpu.

Are there any other ill-effects of sharing an irq across all cpus like
this?  I guess there's some risk of contention on the irq_desc lock.

Thanks,
    J
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