Re: [Qemu-devel] [PATCH] intel_iommu: add "eim" property

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-08-11 21:29+0800, Peter Xu:
> Adding one extra property for intel-iommu device to decide whether we
> should support EIM bit for IR.
> 
> Now we are throwing high 24 bits of dest_id away directly. This will
> cause interrupt issues with guests that:
> 
> - enabled x2apic with cluster mode
> - have more than 16 vcpus (so will have more than 1 cluster)

Good catch, it is a problem starting at 9 VCPUs, because that one is
already "0x100" and we truncate to 8 bits ...

> Let's make xapic the default one, and for the brave people who would
> like to try EIM and know the side effects, we can do it by explicitly
> enabling EIM using:

This might bite us: users cannot easily tell when EIM is sane, so eim=on
is going to be a hazard even after some KVM/QEMU has it fixed.
I'd make make eim=on fail when logical x2APIC is broken.

Right now, we can allow eim=on if maximal APIC ID < 8, because physical
0xffffffff gets truncated to 0xff and is still interpreted as a
broadcast thanks to the quirk.
Logical 0xff gets misinterpreted as a broadcast, but it would address
all APICs anyway and KVM's lowest-priority works for broadcast too, so
it is fine to do so.  Not very wise, though. :)

>   -device intel-iommu,intremap=on,eim=on
> 
> Even after we have x2apic support, it'll still be good if we can provide
> a way to switch xapic/x2apic from QEMU side for e.g. debugging purpose,
> which is an alternative for tuning guest kernel boot parameters.
> 
> We can switch the default to "on" after x2apic fully supported.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> This will have small conflict with Radim's patches to selectively
> enable EIM. I can do a rebase when needed.

This patch made me realize that broadcast quirk is not enough, so I'll
do v2 on top of this.