Re: [PATCH 0/4] Expose the isa-string via the AT_BASE_PLATFORM aux vector

Björn Töpel
2023-05-01T19:55:25Z

Heiko Stuebner <heiko@sntech.de> writes:

> From: Heiko Stuebner <heiko.stuebner@vrull.eu>
>
> The hwprobing infrastructure was merged recently [0] and contains a
> mechanism to probe both extensions but also microarchitecural features
> on a per-core level of detail.
>
> While discussing the solution internally we identified some possible issues,
> tried to understand the underlying issue and come up with a solution for it.
> All these deliberations overlapped with hwprobing being merged, but that
> shouldn't really be an issue, as both have their usability - see below.
> Also please see the "Things to consider" at the bottom!
>
>
> Possible issues:
> - very much limited to Linux
> - schedulers run processes on all cores by default, so will need
> the common set of extensions in most cases

...which hwprobe has support for via the CPU mask. no?

> - each new extensions requires an uapi change, requiring at least
> two pieces of software to be changed
> - adding another extension requires a review process (only known
> extensions can be exposed to user-space)
> - vendor extensions have special needs and therefore possibly
> don’t fit well
>
>
> Limited to Linux:
> -----------------
>
> The syscall and its uapi is Linux-specific and other OSes probably
> will not defer to our review process and requirements just to get
> new bits in. Instead most likely they'll build their own systems,
> leading to fragmentation.

There are a number of examples where multiple OSs have followed what
Linux does, and vice versa. I'd say the opposite -- today system
builders do not do their own solution, but review what's out there and
mimics existing ones.

Personally I think this argument is moot, and will not matter much for
fragmentation.

> Feature on all cores:
> ---------------------
>
> Arnd previously ([1]) commented in the discussion, that there
> should not be a need for optimization towards hardware with an
> asymmetric set of features. We believe so as well, especially
> when talking about an interface that helps processes to identify
> the optimized routines they can execute.
>
> Of course seeing it with this finality might not take into account
> the somewhat special nature of RISC-V, but nevertheless it describes
> the common case for programs very well.
>
> For starters the scheduler in its default behaviour, will try to use any
> available core, so the standard program behaviour will always need the
> intersection of available extensions over all cores.
>
>
> Limiting program execution to specific cores will likely always be a
> special use case and already requires Linux-specific syscalls to
> select the set of cores.
>
> So while it can come in handy to get per-core information down the road
> via the hwprobing interface, most programs will just want to know if
> they can use a extension on just any core.
>
>
> Review process:
> ---------------
>
> There are so many (multi-letter-)extensions already with even more in
> the pipeline. To expose all of them, each will require a review process
> and uapi change that will make defining all of them slow as the
> kernel needs patching after which userspace needs to sync in the new
> api header.
>
>
> Vendor-extensions:
> ------------------
>
> Vendor extensions are special in their own right.
> Userspace probably will want to know about them, but we as the kernel
> don't want to care about them too much (except as errata), as they're
> not part of the official RISC-V ISA spec.
>
> Getting vendor extensions from the dt to userspace via hwprobe would
> require coordination efforts and as vendors have the tendency to invent
> things during their development process before trying to submit changes
> upstream this likely would result in conflicts with assigned ids down
> the road. Which in turn then may create compatibility-issues with
> userspace builds built on top of the mainline kernel or a pre-
> existing vendor kernel.
>
> The special case also is that vendor A could in theory implement an
> extension from vendor B. So this would require to actually assign
> separate hwprobe keys to vendors (key for xthead extensions, key for
> xventana extensions, etc). This in turn would require vendors to
> come to the mainline kernel to get assigned a key (which in reality
> probably won't happen), which would then make the kernel community
> sort of an id authority.
>
>
>
>
> To address these, the attached patch series adds a second interface
> for the common case and "just" exposes the isa-string via the
> AT_BASE_PLATFORM aux vector.

*A second interface* introduced the second hwprobe landed. Really?
Start a discussion on how to extend hwprobe instead.

Björn