Re: [Qemu-devel] Aspirant for AMD IOMMU emulation project for Outreachy

[Jan Kiszka]

Hi Rita,

[CC'ing Andrew due to overlap with split irqchip topic]

On 2015-09-09 19:41, Rita Sinha wrote:
> Hi Jan,
> 
>>
>> Most likely than not you'll work on the Intel IOMMU and I would
>> suggest, if you wish to get your feet dirty, just start right away
>> with the Intel IOMMU (In which case Jan could direct you better)
>> instead of the AMD one.
>>
> 
> As suggested by David, I would request you to guide me as to how to
> get started with Intel IOMMU.
> 

You can find the basic problem description and some links here:

http://wiki.qemu.org/Google_Summer_of_Code_2015#VT-d_interrupt_emulation_with_KVM_support

Who much do you know about IOMMUs and interrupt remapping in general?
The spec may provide a certain introduction to the topic, but it also
quickly dives into details. Let me know where you see gaps to build up a
better understanding of the topic.

Implementation-wise, you could start off with the existing interrupt
remapping patches and look into the missing error reporting for IR on
that basis. I've just rebased and pushed a new version. Error reporting
is currently done in form of plain printf. The correct behaviour is
described in the spec. Feel free to nag me with questions on how things
work around interrupt remapping in general and its QEMU integration in
particular.

The goal is to get this upstream eventually. So the current design needs
to be prepared for submission, reviewed by subsystem maintainer, and
then possibly reworked based on their feedback. We will also need some
command line switch (machine property, like iommu=on) to control the
availability of IR because the original Q35 chipset didn't include this
feature.

But there is also the aim to integrate it with KVM support, and that's
why I'm CC'ing Andrew. At Google, they are currently working on QEMU
patches for their split KVM irqchip model. This will push the IOAPIC
into QEMU hands, even when KVM in-kernel APIC acceleration is used. And
that will allow IR to be installed easily, without any KVM kernel-side
changes.

Andrew, what is the status of your QEMU patches? Are there chances to
preview them in order to asses if and how much integration work is
needed for interrupt remapping?

Rita, you probably noticed that this is no beginner's task, both
regarding the hardware background as well as the QEMU subsystems. You
have worked with several low-level software components before and have
some QEMU experience, so this topic could be a good match for you.
Still, if you have doubts after looking into details, let me know so
that we can discuss them.

Best regards,
Jan

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