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Date: Tue, 30 Jul 2013 12:44:12 +0200
From: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20130730104412.GD14452@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v3] e1000: add interrupt mitigation support
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To: Vincenzo Maffione <v.maffione@gmail.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>, "Michael S. Tsirkin" <mst@redhat.com>, Luigi Rizzo <rizzo@iet.unipi.it>, Jason Wang <jasowang@redhat.com>, qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>, Giuseppe Lettieri <g.lettieri@iet.unipi.it>, lersek@redhat.com, Andreas =?iso-8859-1?Q?F=E4rber?= <afaerber@suse.de>

On Mon, Jul 29, 2013 at 04:46:22PM +0200, Vincenzo Maffione wrote:
> Actually it's not always zero because
> 
> - presave() calls e1000_mit_timer(), which sets mit_timer_on to 0 and
> calls set_interrupt_cause(ICR);
> 
> - if there are pending events, set_interrupt_cause(ICR) detects a
> raising edge, and doesn't filter out the interrupt since mit_timer_on
> == 0. Therefore, the timer is reloaded and mit_timer_on is set to 1.
> 
> However, I've just realized that this is a mistake, since it would
> need the post_load() to call qemu_mod_timer() when
> mit_timer_on is migrated as "1", otherwise the interrupt delivery
> would deadlock on the migrated system.
> 
> I think it's way better not to migrate mit_timer_on, so that the
> migrated system sees mit_timer_on==0  and doesn't need to call
> qemu_mod_timer() in post_load(). The first interrupt request
> (set_interrupt_cause()) will cause an immediate interrupt.

Thanks for explaining.  I agree that it's simpler to avoid migrating
mit_timer_on.

Stefan