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

[Radim Krčmář]

2016-04-26 17:28+0200, 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.

I agree, both of them should change.  KVM_SIGNAL_MSI was added just
because the route-based injection in kvm_irqchip_send_msi() sucked;
maybe we could find a good solution now, but the direct interface is
already here ...