On Tue, May 10, 2016 at 03:15:12PM +0100, Peter Maydell wrote:

On 10 May 2016 at 15:04, Christoffer Dall [off-list ref] wrote:

quoted

On Fri, May 06, 2016 at 11:45:32AM +0100, Andre Przywara wrote:

quoted

+     /*
+      * Currently all guest IRQs are Group1, as Group0 would result
+      * in a FIQ in the guest, which it wouldn't expect.

I still don't like or understand this comment.  This should simply say
that we're making a gross assumption about all interrupts being group1
here.

It's not really an assumption so much as it's a missing feature (aka bug):
there's no reason the vGIC shouldn't support group 0 interrupts. We
just get away with only supporting group 1 because Linux guests
happen to only use group 1 interrupts. If/when the vGIC gains support
for group0 interrupts, then it should reset with interrupts configured
in group0 by default.

quoted

quoted

+      * Eventually we want to make this configurable, so we may
+      * revisit this in the future.
+      */

The only reason to make it configurable is to work around a guest
kernel bug whereby Linux assumes that all interrupts start out
in Group1. Marc sent out a patch earlier today that fixes that bug:
 https://lkml.org/lkml/2016/5/10/297
Depending on how long it takes you to fix this missing vgic feature,
such kernels may all be long-forgotten, in which case you can
get away without the config option :-)

Agreed with all of the above.  My nit here is simply that if we don't
implement grouping support now, then it's just a gross hack, and we
should just state that until we fix it properly.

-Christoffer