[Qemu-devel] QEMU GSoC 2018 Project Idea (Apply polling to QEMU NVMe)

[Huaicheng Li]

Hi all,

The project would be about utilizing shadow doorbell buffer features in
NVMe 1.3 to enable QEMU side polling for virtualized NVMe device, thus
achieving comparable performance as in virtio-dataplane.

**Why not virtio?**
The reason is many industrial/academic researchers uses QEMU NVMe as a
performance platform for research/product prototyping. NVMe interface is
better in the rich features it provides than virtio interface. If we can
make QEMU NVMe performance competent with virtio, it will benefit a lot of
communities.

**Doable?**
NVMe spec 1.3 introduces a shadow doorbell buffer which is aimed for
virtual NVMe controller optimizations. QEMU can certainly utilize this
feature to reduce or even eliminate VM-exits triggered by doorbell writes.

I remember there were some discussions back in 2015 about this, but I don't
see it finally done. For this project, I think we can go in three steps:
(1). add the shadow doorbell buffer support into QEMU NVMe emulation, this
will reduce # of VM-exits. (2). replace current timers used by QEMU NVMe
with a separate polling thread, thus we can completely eliminate VM-exits.
(3). Even further, we can adapt the architecture to use one polling thread
for each NVMe queue pair, thus it's possible to provide more performance.
(step 3 can be left for next year if the workload is too much for 3 months).

Actually, I have an initial implementation over step (1)(2) and would like
to work more on it to push it upstream. More information is in this papper,
(Section 3.1 and Figure 2-left),
http://ucare.cs.uchicago.edu/pdf/fast18-femu.pdf

Comments are welcome.

Thanks.

Best,
Huaicheng