Re: [PATCH v5 23/23] xen/README: add compiler and binutils versions for RISC-V64

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 29/02/2024 12:51, Jan Beulich wrote:
> On 29.02.2024 13:32, Julien Grall wrote:
> > On 29/02/2024 12:17, Jan Beulich wrote:
> > > On 29.02.2024 13:05, Andrew Cooper wrote:
> > > > On 29/02/2024 10:23 am, Julien Grall wrote:
> > > > > > > > IOW it is hard for me to see why RISC-V needs stronger restrictions
> > > > > > > > here
> > > > > > > > than other architectures. It ought to be possible to determine a
> > > > > > > > baseline
> > > > > > > > version. Even if taking the desire to have "pause" available as a
> > > > > > > > requirement, gas (and presumably gld) 2.36.1 would already suffice.
> > > > > > > I think we want to bump it on Arm. There are zero reasons to try to
> > > > > > > keep
> > > > > > > a lower versions if nobody tests/use it in production.
> > > > > > >
> > > > > > > I would suggest to do the same on x86. What's the point of try to
> > > > > > > support Xen with a 15+ years old compiler?
> > > > > > It could have long been bumped if only a proper scheme to follow for
> > > > > > this and future bumping would have been put forward by anyone keen on
> > > > > > such bumping, like - see his reply - e.g. Andrew. You may recall that
> > > > > > this was discussed more than once on meetings, with no real outcome.
> > > > > > I'm personally not meaning to stand in the way of such bumping as long
> > > > > > as it's done in a predictable manner, but I'm not keen on doing so and
> > > > > > hence I don't view it as my obligation to try to invent a reasonable
> > > > > > scheme. (My personal view is that basic functionality should be
> > > > > > possible to have virtually everywhere, whereas for advanced stuff it
> > > > > > is fine to require a more modern tool chain.)
> > > > > That's one way to see it. The problem with this statement is a user
> > > > > today is mislead to think you can build Xen with any GCC versions
> > > > > since 4.1. I don't believe we can guarantee that and we are exposing
> > > > > our users to unnecessary risk.
> > > > >
> > > > > In addition to that, I agree with Andrew. This is preventing us to
> > > > > improve our code base and we have to carry hacks for older compilers.
> > > > I don't think anyone here is suggesting that we switch to a
> > > > bleeding-edge-only policy.  But 15y of support is extreme in the
> > > > opposite direction.
> > > >
> > > > Xen ought to be buildable in the contemporary distros of the day, and I
> > > > don't think anyone is going to credibly argue otherwise.
> > > >
> > > > But, it's also fine for new things to have newer requirements.
> > > >
> > > > Take CET for example.  I know we have disagreements on exactly how it's
> > > > toolchain-conditionalness is implemented, but the basic principle of "If
> > > > you want shiny new optional feature $X, you need newer toolchain $Y" is
> > > > entirely fine.
> > > >
> > > > A brand new architecture is exactly the same.  Saying "this is the
> > > > minimum, because it's what we test" doesn't preclude someone coming
> > > > along and saying "can we use $N-1 ?  See here it works, and here's a
> > > > change to CI test it".
> > > >
> > > >
> > > > Anyway, its clear we need to write some policy on this, before making
> > > > specific adjustments.  To get started, is there going to be any
> > > > objection whatsoever on some principles which begin as follows:
> > > Largely not, but one aspect needs clarifying up front:
> > >
> > > > * For established architectures, we expect Xen to be buildable on the
> > > > common contemporary distros.  (i.e. minima is not newer than what's
> > > > available in contemporary distros, without a good reason)
> > > What counts as contemporary distro? Still in normal support? LTS? Yet
> > > more extreme forms?
> > LTS makes sense. More I am not sure. I am under the impression that
> > people using older distros are those that wants a stable system. So they
> > would unlikely try to upgrade the hypervisor.
> >
> > Even for LTS, I would argue that if it has been released 5 years ago,
> > then you probably want to update it at the same time as moving to a
> > newer Xen version.
> For the purposes of distros I agree. For the purposes of individuals
> I don't: What's wrong with running a newer hypervisor and/or kernel
> underneath an older distro?
There is nothing wrong. I just don't understand the benefits for us to 
support that use case. To me there are two sorts of individuals:
 1. The ones that are using distro packages. They will unlikely want to 
switch to a newer hypervisor
 2. The ones that are happy to compile and hack their system. Fairly 
likely they will use a more distros and/or would not be put up by 
upgrading it.
What individuals do you have in mind?

Also, for me, the minimum doesn't prevent anyone to try to compile with 
an older compiler. It is only here to say that as a community we will 
not investigate or trying to workaround bugs in those compilers.
I don't see the problem with that if someone decide to use an older dom0 
distros with a newer hypervisor. It would likely not be the only issue 
they will have.
Cheers,

--
Julien Grall