On Sat, 4 Dec 2010, Jan Kiszka wrote:
> Besides 3 cleanup patches, this series consists of two major changes.
> The first introduces an interrupt sharing notifier to the genirq
> subsystem. It fires when an interrupt line is about to be use by more
> than one driver or the last but one user called free_irq.
>
> The second major change makes use of this interface in KVM's PCI pass-
> through subsystem. KVM has to keep the interrupt source disabled while
> calling into the guest to handle the event. This can be done at device
> or line level. The former is required to share the interrupt line, the
> latter is an order of magnitude faster (see patch 3 for details).
>
> Beside pass-through support of KVM, further users of the IRQ notifier
> could become VFIO (not yet mainline) and uio_pci_generic which have to
> resolve the same conflict.
Hmm. You basically want to have the following functionality:
If interrupt is shared, then you want to keep the current behaviour:
disable at line level (IRQF_ONESHOT)
run handler thread (PCI level masking)
reenable at line level in irq_finalize_oneshot()
reenable at PCI level when guest is done
If interrupt is not shared:
disable at line level (IRQF_ONESHOT)
run handler thread (no PCI level masking)
no reenable at line level
reenable at line level when guest is done
I think the whole notifier approach including the extra irq handlers
plus the requirement to call disable_irq_nosync() from the non shared
handler is overkill. We can be more clever.
The genirq code knows whether you have one or more handler
registered. So we can add IRQF_ONESHOT_UNMASK_SHARED and add a status
field to irq_data (which I was going to do anyway for other
reasons). In that status field you get a bit which says IRQ_MASK_DEVICE.
So with IRQF_ONESHOT_UNMASK_SHARED == 0 we keep the current behaviour.
If IRQF_ONESHOT_UNMASK_SHARED== 1 and IRQ_MASK_DEVICE == 1 we keep the
current behaviour.
If IRQF_ONESHOT_UNMASK_SHARED== 1 and IRQ_MASK_DEVICE == 0 then then
irq_finalize_oneshot() simply marks the interrupt disabled (equivalent
to disable_irq_nosync()) and returns.
Now in your primary irq handler you simply check the IRQ_MASK_DEVICE
status flag and decide whether you need to mask at PCI level or not.
Your threaded handler gets the same information via IRQ_MASK_DEVICE so
it can issue the appropriate user space notification depending on that
flag.
This works fully transparent across adding and removing handlers. On
request_irq/free_irq we update the IRQ_MASK_DEVICE flag with the
following logic:
nr_actions IRQF_ONESHOT_UNMASK_SHARED IRQ_MASK_DEVICE
1 0 1
1 1 0
>1 don't care 1
If interrupts are in flight accross request/free then this change
takes effect when the next interrupt comes in.
No notifiers, no disable_irq_nosync() magic, less and simpler code.
Thoughts ?
tglx
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