Re: [PATCH v2 2/6] RISC-V: Add a syscall for HW probing

[Evan Green]

On Thu, Feb 16, 2023 at 5:30 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Wed, Feb 15, 2023, at 23:43, Jessica Clarke wrote:
> > On 15 Feb 2023, at 21:14, Evan Green <evan@xxxxxxxxxxxx> wrote:
> >> On Wed, Feb 15, 2023 at 1:57 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> >> Palmer can probably speak to this with more authority, but my
> >> understanding about the motivation for an approach like this goes
> >> something like:
> >> * With the nature of RISC-V, we expect a lot of these types of bits
> >> and bobs, many more than we've seen with the likes of x86 and ARM.
> >
> > We’re already at (I think) 51 standard user-level extensions that LLVM
> > knows about.
>
> Do you have an estimate of how many of these require kernel support
> beyond identifying the extensions?
>
> >> * We also expect in some cases these values to be inconsistent across CPUs.
> >
> > That’s also true of some Arm SoCs.
>
> Right, but it's also something that we should not encourage, or
> need to make easy to use. On arm64, the kernel support for having
> asymmetric aarch32 mode was kept to an absolute minimum, and an
> application is expected to get the information from /proc/cpuinfo
> before pinning down a task to the correct subset of all CPUs.
>
> >> * So, a syscall with a vDSO function in front of it seemed like a
> >> good combination of speed and flexibility.
> >>
> >> You're certainly right that HWCAPn would work for what we're exposing
> >> today, so the question probably comes down to our relative predictions
> >> of how this data will grow.
> >
> > The other big problem is vendor extensions.

Since the key range can grow without accruing a process startup time
penalty, this proposal handles vendor extensions fairly well, no?
(Contrasting at least against hwcap bits, which once allocated have to
be copied into every new process forever).

>
> My biggest concern is how this would be synchronized between the
> interfaces that are available to users. What we have on other architectures
> is a set of string identifiers in /proc/cpuinfo and a bitmask in HWCAP.
> Ideally these are added in pairs so the information available to shell
> scripts in human readers is the same that is available in the auxvec
> data.
>
> Adding a third interface with the same information or a superset
> requires more work in ensuring that each extension is available
> in exactly the right places. Ideally I think there should be only
> one table of possible CPU features so nobody has to make the
> decision about which ones are important enough to add to one
> interface or another.

That makes sense. In this case, the proposal is that RISC-V would use
this mechanism and generally abandon hwcap. So my hope is there should
still only be about 2 spots to maintain.
-Evan