RE: [EXTERNAL] RE: [PATCH v2 0/4] Add new headers for Hyper-V Dom0

Michael Kelley
2024-11-11T23:41:15Z

From: MUKESH RATHOR <mukeshrathor@microsoft.com> Sent: Monday, November 11, 2024 3:11 PM
>
> On 11/11/24 13:28, Michael Kelley wrote:
> > From: MUKESH RATHOR <mukeshrathor@microsoft.com> Sent: Monday, November 11, 2024 10:53 AM
> >>
> >> On 11/10/24 20:12, Michael Kelley wrote:
> >> > From: Nuno Das Neves <nunodasneves@linux.microsoft.com> Sent:
> >> Thursday, November 7, 2024 2:32 PM
> >> >>
> >> >> To support Hyper-V Dom0 (aka Linux as root partition), many new
> >> >> definitions are required.
> >> >
> >> > Using "dom0" terminology here and in the Subject: line is likely to
> >> > be confusing to folks who aren't intimately involved in Hyper-V work.
> >> > Previous Linux kernel commit messages and code for running in the
> >> > Hyper-V root partition use "root partition" terminology, and I couldn't
> >> > find "dom0" having been used before. "root partition" would be more
> >> > consistent, and it also matches the public documentation for Hyper-V.
> >> > "dom0" is Xen specific terminology, and having it show up in Hyper-V
> >> > patches would be confusing for the casual reader. I know "dom0" has
> >> > been used internally at Microsoft as shorthand for "Hyper-V root
> >> > partition", but it's probably best to completely avoid such shorthand
> >> > in public Linux kernel patches and code.
> >> >
> >> > Just my $.02 ....
> >>
> >> Hi Michael,
> >>
> >> FWIW, hyperv team and us are using the term "dom0" more and more to
> >> avoid confusion between windows root and linux root, as dom0 is
> >> always linux root. I did a quick search, and "dom0" is neither
> >> copyrighted nor trademarked by xen, and I'm sure the fine folks
> >> there won't be offended. Hopefully, [Hyper-V] tag would reduce
> >> the confusion.
> >>
> >> Just my $0.1
> >>
> >
> > Yeah, "dom0" certainly fits as shorthand for the rather ponderous
> > "Linux running in a Hyper-V root partition". :-)
> >
> > But even using "Hyper-V dom0" to add clarity vs. Xen dom0 seems
> > to me to be a misnomer because Hyper-V dom0 is only conceptually
> > like Xen dom0. It's not actually an implementation of Xen dom0.
> > Let me give two examples:
> >
> > 1) Hyper-V provides VMBus, which is conceptually similar to virtio.
> > But VMBus is not an implementation of virtio, and we don't call it
> > "Hyper-V virtio". Of course, "VMBus" is a lot shorter than "Hyper-V
> > root partition" so the motivation for a shorthand isn't there, but still.
> > If Hyper-V should ever implement actual virtio interfaces, then it
> > would be valid to call that "Hyper-V virtio".
> >
> > 2) KVM has "KVM Hyper-V", which I think is valid. It's an
> > implementation of Hyper-V interfaces in KVM so that Windows
> > guests can run as if they are running on Hyper-V.
> >
> > I won't speculate on what the Xen folks would think of "Hyper-V
> > dom0", especially if it isn't an implementation that's compatible
> > with Xen dom0 functionality.
> >
> > As for "more and more" usage of "dom0" by your team and the
> > Hyper-V team: Is that internal usage only? Or usage in public mailing
> > lists or open source projects like Cloud Hypervisor? Again, from
> > my standpoint, internal is internal and can be whatever is convenient
> > and properly understood internally. But in public mailing lists and
> > projects, I think "Hyper-V dom0" should be avoided unless it's
> > truly an implementation of the dom0 interfaces.
> >
> > That's probably now $0.10 worth instead of $0.02. :-) And I'm
> > not the decider here -- I'm just offering a perspective.
>
> "dom0" is neither a technology nor a protocol. It simply means initial
> domain (which on xen happened to be domid of 0, could have been 1). This
> is created during boot, same as linux root on hyperv, and is privileged
> domain same as xen. Even in KVM world, I've heard many folks refer to
> the host as kvm dom0...
>
> Given the mix of windows and linux with l1vh and nested, dom0 is helping
> in conversations internally, and I'm sure it will keep percolating
> externally.
>

OK, fair enough. My perspective is probably more limited than
yours as my experience with "dom0" is exclusively with the Xen
code in a Linux kernel environment. I just haven't seen "dom0"
used elsewhere, but that certainly doesn't mean it's not being
done.

If the decision on the Microsoft side is that introducing "Hyper-V
dom0" terminology makes sense, I won't object further, though
I would think there should be some kind of reconciliation with
existing code/comments/documentation that uses "root partition".

Michael