Re: [Qemu-devel] [PATCH v4 00/16] IOMMU: Enable interrupt remapping for Intel IOMMU

[Radim Krčmář]

2016-04-26 16:24+0200, Jan Kiszka:
> On 2016-04-26 13:40, Peter Xu wrote:
>> Currently, all the interrupts will be translated into one MSI in
>> vtd_generate_msi_message(), in which only 8 bits of dest_id is used
>> (msg.dest = irq->dest). We may possibly need to use the high 32 bits
>> of MSI address to store the rest dest[31:8]? Don't know whether this
>> would be enough though.
> 
> Yes, I ran into this topic as well as I hacked up those line. Currently,
> KVM does not support more than 254 vCPUs, so 8 bits of those 32 are
> actually fine, and piggy-backing them in an MSI message works.
> 
> Once KVM supports more CPUs, it has to come up with a new userspace
> interface to inject APIC events for more than 255 CPUs. Maybe the
> existing direct MSI inject with its unused flags could be "bended",
> maybe there are already better ideas (Radim?).

Adding a flag to msi_msg and taking 3-4 bytes from padding to express
x2APIC addresses is reasonable.  (It is what my prototype did. :])

The conceptually different idea is forcing all userspace interrupts
through irqfd routes, which would obsolete the ad-host inject.

> In any case, the KVM layer in userspace will then have to pick up all 32
> destination bits from the IRTE and deliver them via that new interface
> to the in-kernel APICs.

I'll keep that in mind, thanks.