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

[Jan Kiszka]

On 2016-04-26 16:59, Radim Krčmář wrote:
> 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.

irqfd for userspace sources is a bit clumsy from the API POV. On the
other hand, we need to tweak the routing API anyway to achieve the same
address extension there, too.

Jan

> 
>> 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.
>