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

[Peter Xu]

On Thu, Aug 11, 2016 at 05:44:16PM +0200, Radim Krčmář wrote:
> 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 ...

Ah yes. I muddled on the 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 <address@hidden>
> > ---
> > 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.

I'll post v2 soon with DEFINE_PROP_BOOL. Thanks,

-- peterx