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Re: [Xen-devel] fooey. no interrupts.

To: ron minnich <rminnich@xxxxxxxx>
Subject: Re: [Xen-devel] fooey. no interrupts.
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Mon, 09 Aug 2004 08:28:15 +0100
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> I've just realized a few days ago, when I get back to xen/plan 9, that I'm 
> not getting interrupts after the first few. This with a very recent pull. 
> What's amazing that it got as far as it did, but I am processing pending 
> interrupt stuff in spllo() so that explains a lot. What I'm not getting is 
> the asynchronous calls to evtchn_do_upcall. 
> The mask is zero. I've enabled VIRQ_TIMER. Yet I'm only getting one set of
> interrupts and it looks like no more. My loop for picking up events is
> pretty much the same as the linux loop -- I just took that code. I am 
> clearing out evtchn_upcall_pending and evtchn_pending_sel. I am clearing 
> the mask to 0 at the end of the interrupt. 
> What's a reasonable set of things to look for? I'm stumped.

The Linux code sets the evtchn_mask before clearing evtchn_pending,
then clears the evtchn_mask after calling the interrupt handler. Are
you doing the setting but forgetting the clearing?

The order that Linux has for this stuff, to avoid races, is:

 1. Test-and-clear evtchn_upcall_pending flag
 2. Read-and-clear (XCHG) the evtchn_pending_sel
 3. For each set bit @i in the sel:
 4.   Read evtchn_pending[@i]
 5.   For each set bit @j in the word:
 6.     Set evtchn_mask[@i*32+@j]
 7.     Clear evtchn_pending[@i*32+@j]
 8.     ....do interrupt work...
 9.     Clear evtchn_msk[@i*32+@j]

The fact that step 2 is a real XCHG instruction is important, as it
also acts as a memory barrier (not important if you're not running on
an SMP machine). Also, all your bit-munging instructions must have the
LOCK prefix if you're running on an SMP machine.

Unmasking evtchn_upcall_mask and evtchn_mask[] need special attention
because a pending interrupt will not automatically get raised as it
would on real hardware ---- think of it like an edge-triggered
interrupt where you lost the edge because the line was masked. So what
we do in Linux is:

Clearing evtchn_upcall_mask:
 1. Clear evtchn_upcall_mask
 2. Barrier [just a compiler barrier, not a CPU barrier]
 3. If ( evtchn_upcall_pending) do_evtchn_processing()

Clearing evtchn_mask[]:
 A bit more involved; see unmask_evtchn() in include/asm-xen/evtchn.h

Sticking close to the Linux code, and making sure the underlying
bitops are LOCKed, is important! I guess yours is unlikely to be a
subtle race if you only ever receive precisly one VIRQ. :-)

 -- Keir

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