Re: Xen article for the FreeBSD Journal

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

Hi Roger,

The article is very well written and I really enjoyed reading it. Well
done! I have a couple of minor suggestions, feel free to take them or
ignore them as you see fit.


I would remove the sentence "PVH mode however requires modifications in
the guest OS kernel so it's aware it's running under Xen and some
devices are not available." I am not sure whether that's really true.
Within AMD, we have added HPET emulation (optionally) to PVH, with that
I think an unmodified guest could run as PVH.


In the embedded section just before 'dom0less/hyperlaunch,' it would be
beneficial to highlight the importance of real-time requirements. The
vast majority of embedded deployments rely on low and deterministic
interrupt latency to meet real-time needs. Significant efforts have been
put into Xen to ensure interrupt latency remains both low and
deterministic, even in the presence of cache pressure caused by noisy
neighbors. Notably, the cache coloring patch series, now at version 9,
has demonstrated excellent results in mitigating cache interference and
maintaining deterministic latency at 2.5 microseconds.


This sentence: "There's an ongoing effort in Xen upstream to add PCI
passthrough support for PVH dom0, however that's still being worked on,
and when finished will require changes to FreeBSD for the feature to be
usable." Why do you think there will be work required in FreeBSD? With
vPCI, my expectation is that no work is required in FreeBSD (if the
domUs are also PVH).


The sentence: "There are ongoing changes to the VirtIO specification to
use grants instead of guest memory addresses as the basis for memory
sharing between the VirtIO frontends and backends." The addition of
grant table support to VirtIO is already completed. It is upstream in
both Linux and QEMU. There is nothing else left to do there, and there
is no need for a spec change. You can highlight that. At the same time
it is true that there is still ongoing work to introduce changes to the
VirtIO spec to make it safer and more secure (VirtIO-msg).


Just before 'The future of Xen,' and following the discussion on MISRA
C, it is worth noting that we have recently begun upstreaming safety
requirements and assumptions of use. These can be found in
docs/fusa/reqs. Safety requirements provide a detailed description of
all the expected behaviors of the software (Xen), enabling independent
testing and validation of these behaviors.

Cheers,
Stefano


On Thu, 14 Nov 2024, Roger Pau Monné wrote:
> Hello,
> 
> I've been asked to write a Xen article for the FreeBSD Journal [0], a
> free FreeBSD magazine published by the FreeBSD Foundation.
> 
> The request was something about Xen for an upcoming virtualization
> focused issue, but it could can be a status report, new features on
> Xen, or even a tutorial about how to use Xen on FreeBSD.
> 
> I've decided to do an article about the Xen architecture, specific
> features that make Xen different from other hypervisors, plus some
> FreeBSD background about how Xen support is implemented in FreeBSD.
> 
> I feel like I'm missing a bit more ARM content to balance the x86
> stuff, but sadly I'm not sure what's worth highlighting there, so any
> recommendations are welcome.
> 
> I'm attaching an early draft for the article to this email in plain
> text.  Note the diagrams will be re-done for them to match the style
> of the publication, and are currently hand drawings since I didn't
> want to waste time on something that will be redone anyway by the
> layout editor.
> 
> I've been told the article should be submitted by the middle of
> November, I'm a bit short on time.  I would like to give them
> something on Monday so they also have time to provide feedback.
> 
> Thanks, Roger.
> 
> [0] https://freebsdfoundation.org/our-work/journal/
>