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

[Jan Kiszka <jan.kiszka@web.de>]

On 2016-04-25 09:18, Peter Xu wrote:
> On Mon, Apr 25, 2016 at 07:16:19AM +0200, Jan Kiszka wrote:
>> On 2016-04-19 10:38, Peter Xu wrote:
> 
> [...]
> 
>>> By default, IR is disabled to be better compatible with current
>>> QEMU. To enable IR, we can using the following command to boot a
>>> IR-supported VM with virtio-net device with vhost (still do not
>>> support kvm-ioapic, so we need to specify kernel-irqchip={split|off}
>>> here):
>>>
>>> $ qemu-system-x86_64 -M q35,iommu=on,intr=on,kernel-irqchip=split \
>>
>> "intr" sounds a bit too much like "interrupt", not "interrupt
>> remapping". Why not use the kernel's form, "intremap"?
> 
> Sure. It sounds nice to be aligned with the kernel one. Let me take
> it in v5.
> 
>>
>>>      -enable-kvm -m 1024 \
>>> 	 -netdev tap,id=net0,vhost=on \
>>>      -device virtio-net-pci,netdev=user.0 \
>>>      -monitor telnet::3333,server,nowait \
>>> 	 /var/lib/libvirt/images/vm1.qcow2
>>>
>>> When guest boots, we can verify whether IR enabled by grepping the
>>> dmesg like:
>>>
>>> [root@localhost ~]# journalctl -k | grep "DMAR-IR"
>>> Feb 19 11:21:23 localhost.localdomain kernel: DMAR-IR: IOAPIC id 0 under DRHD base  0xfed90000 IOMMU 0
>>> Feb 19 11:21:23 localhost.localdomain kernel: DMAR-IR: Enabled IRQ remapping in xapic mode
>>>
>>> Currently supported devices:
>>>
>>> - Emulated/Splitted irqchip
>>> - Generic PCI Devices
>>> - vhost devices
>>> - pass through device support? Not tested, but suppose it should work.
>>
>> I've tested this series against my Jailhouse setup, and it works pretty
>> well! Actually considering to move my test setup over this branch.
> 
> This is really encouraging feedback! Btw, thanks for all kinds of
> help on this patchset. :-)
> 
>>
>> However, split irqchip still has some issues: When I boot a q35 machine
>> with Linux, the e1000 network adapter only gets a single IRQ delivered.
>> Interestingly, other IOAPIC IRQs like the keyboard work all the time. I
>> didn't debug this in details yet.
> 
> I reproduced this problem. It seems that it fails even with
> kernel-irqchip=off. Will try to dig it out.

Very good. Hope it can be easily fixed.

> 
>>
>>>
>>> TODO List:
>>>
>>> - kvm-ioapic support (?)
>>> - EIM support
>>
>> That should be fairly easy, I already played with it (hack in EIM cap,
>> change vtd_remap_irq_get, assuming EIME would be set). However, it
>> depends on split irqchip to work properly (there is no x2apic in
>> userspace APIC), and that is not yet the case.
> 
> That's cool. Never tried it though. Anyway, will leave x2apic
> related work for Radim. :)

Whoever is faster ;) - I would really like to have this feature in order
to test x2apic support of Jailhouse also in the virtual test bed. Will
surely hack on it as soon as that other bug is fixed.

> 
>>
>>> - IR fault reporting
>>
>> Would be welcome! I found a "test case" yesterday: misconfigured IOAPIC
>> ID blocked its IRQs under Jailhouse, and I first had to enable tracing
>> to realize it ;).
> 
> Yes, it sounds nice to have guest side feedback on IR faults. Will
> do more reading, and see whether I can add one more patch in v5 to
> do this.

It's not a must-have for getting things merged. In fact, any additional
feature that could now delay the merge of what you have should rather
wait. Stabilizing, addressing style and structure comments is more
important IMO.

Jan