[linux-kvm B/F/G/H][PATCH 0/1] Enable CONFIG_PCI_MSI in the linux-kvm derivative

[Guilherme G. Piccoli]

BugLink: https://bugs.launchpad.net/bugs/1914283


[Impact]
* Currently linux-kvm derivative doesn't have CONFIG_PCI_MSI (and its dependency
options) enabled. The goal for such derivative is to be minimal and boot as fast as
possible in virtual environments, hence most config options were dropped.

* Happens that MSI/MSI-X are the de facto drivers' standard with regards to interrupts,
and as such the hot path is optimized for MSIs. Boot testing with that config
enabled showed that we have improvements in boot time (details in next section).

* Also, performance-wise MSIs are a good idea too, since it usually allows multiple
queues in network devices and KVM is more optimized to MSIs in comparison with regular
IRQs - tests (detailed in next section) showed performance improvements in virtio
devices with MSIs.

* Based on that findings, we are hereby enabling MSIs for the linux-kvm derivatives
in all series (Bionic / Focal / Groovy / Hirsute) - notice that Xenial already has
that config option enabled.

[Test Case]
* All below tests were performed in a x86-64 KVM guest with 2 VCPUs and 2GB of RAM,
running in a Focal host. Three runs of each test were performed, and we took the average.

* Boot time test (measured by dmesg timestamp) showed an improvement of ~21%,
the following chart exhibiting the data: https://kernel.ubuntu.com/~gpiccoli/MSI/boot_time.svg
We also timed the full boot until the login prompt is available, we had a decrease from ~1 second.

* The storage test was performed with the fio tool, using a virtio-blk empty disk.
The following arguments were used:
fio --filename /dev/vdc --rw=rw --runtime 600 --loops 100 --ioengine libaio --numjobs 2 --group_reporting

On average we had a ~4.5% speedup in both reads and writes, the following chart
represents the data: https://kernel.ubuntu.com/~gpiccoli/MSI/fio_storage.svg

* From the network perspective, we've used iPerf with the following arguments:
iperf -c <server> -t 300 (server was the host machine). On average, the performance
improvement was ~8%, as per the following chart: https://kernel.ubuntu.com/~gpiccoli/MSI/iperf_network.svg


[Where problems could occur]
* Given that the main linux package (generic) and basically all other derivatives
already enable this option, and given that MSIs are the standard with regards to interrupts
from drivers point-of-view, it's safe to say the risks are minimal, likely smaller than
not enabling MSIs (since the hot path is usually more tested/exercised).

* That said, problems could occur if we have bugs in MSI-related code in drivers or in
PCI MSI core code, then those potential problems that would already affect all other
derivatives begin to affect linux-kvm with this change.


Guilherme G. Piccoli (1):
  UBUNTU: [Config] kvm: enable PCI_MSI

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2.29.0