Re: [PATCH v12 0/7] RISC-V: ifunced memcpy using new kernel hwprobe interface

[enh <enh@google.com>]

On Thu, Feb 15, 2024 at 8:50 AM Palmer Dabbelt <palmer@rivosinc.com> wrote:
>
> On Thu, 15 Feb 2024 07:48:03 PST (-0800), Evan Green wrote:
> > On Wed, Feb 14, 2024 at 10:16 AM Adhemerval Zanella Netto
> > <adhemerval.zanella@linaro.org> wrote:
> >>
> >>
> >>
> >> On 14/02/24 11:31, Evan Green wrote:
> >> >
> >> > This series illustrates the use of a recently accepted Linux syscall that
> >> > enumerates architectural information about the RISC-V cores the system
> >> > is running on. In this series we expose a small wrapper function around
> >> > the syscall. An ifunc selector for memcpy queries it to see if unaligned
> >> > access is "fast" on this hardware. If it is, it selects a newly provided
> >> > implementation of memcpy that doesn't work hard at aligning the src and
> >> > destination buffers.
> >> >
> >> > For applications and libraries outside of glibc that want to use
> >> > __riscv_hwprobe() in ifunc selectors, this series also sends a pointer
> >> > to the riscv_hwprobe() function in as the second argument to ifunc
> >> > selectors. A new inline convenience function can help application and
> >> > library callers to check for validity and quickly probe a single key.
> >>
> >> I still think we should address Jessica Clarke remarks for the ifunc ABI [1].
> >> I recall that Florian has tried to address the ifunc ordering and that
> >> Jessica proposed solutions was not fully sufficient to address all the
> >> ifunc corner cases.
> >>
> >> [1] https://sourceware.org/pipermail/libc-alpha/2024-January/154082.html
> >
> > I haven't invested the time yet in studying the resolver to understand
> > how feasible Jessica's suggestion is. I was sort of hoping Florian
> > would chime in with an "oh yeah let's do that" or "no, it doesn't
> > work". I suppose I still am :)
>
> The discussion's over here:
> https://inbox.sourceware.org/libc-alpha/20231017044641.pw2ccr6exvhtmhkk@google.com/
>
> I was inclined to just ignore it: Florian explained what's going on with
> the multiple libraries constraint, so we're kind of just going around in
> circles at that point.
>
> I'm also not sure the argument makes a whole lot of sense in the first
> place.  The core of the argument seems to be around binary compatibility
> with other kernels/libcs, but there's a ton of reasons why binaries
> aren't compatible between those systems.  So if glibc just has a more
> complex IFUNC resolving scheme and that ends up requiring the extra
> agrument, then users of glibc have to go deal with that -- other
> systems/libcs might make different decisions, but that's just how these
> things go.

yeah, i struggle with the portability premise too. not least because
macOS/iOS (which is obviously the most common "other platform" for me
with my Android hat on) doesn't have ifuncs at all. that's quite a big
blocker to any dream of portability.

as i've said before, in a survey of all the open source libraries that
go into Android, there are none that don't support macOS/iOS, and so
there are none that actually rely on ifuncs. ifunc usage is limited to
libc (which is why we libc maintainers get bikeshedded by stuff like
this) and compiler-generated FMV stuff. i'd argue that "real people"
(app developers) should probably be looking at the latter anyway, and
it's our job to make that work (which happens once^Wtwice, in llvm and
gcc).

also, the fact that Android is doing what's proposed here (with the
extra argument) means there'd be _some_ incompatibility even if glibc
and FreeBSD didn't do that.

(Android's only source incompatibility with glibc at the moment is the
lack of the single-probe helper inline, which i fear will encourage
_worse_ code, but suspect will actually be unused in practice anyway
for the same "ifuncs are 'assembler' for libc/toolchain, and
library/app developers doing stuff themselves will continue to use
regular function pointers like they do today" reasons.)

> Looks like Maskray is proposing making glibc's IFUNC resolving.  That's
> a generic glibc decision and I don't really understand this stuff well
> enough to have much of an opinion -- sure maybe the multi-library IFUNC
> resolving is overkill, but aside from doing some similar grep of all the
> sources (or trying some builds and such) I'm not sure how to tell if
> we're safe breaking the ABI there.
>
> That said, we're not comitting to ABI stability until the release.  So
> if we decide to make a generic glibc change we can always go drop the
> argument, that should be easy.  Maybe we even just throw out a RFC patch
> to do it, unless I'm missing something the code is the easy part here
> (we're essentially just going back to one of the early versions, from
> before we knew about this resolving order complexity).
>
> > Alternatively, patches 1-3 of this series stand on their own. If the
> > ifunc aspect of this is gated on me doing a bunch of research, it
> > might at least make sense to land the first half now, to get Linux
> > users easy access to the __riscv_hwprobe() syscall and vDSO.
>
> I'm OK with that, too.
>
> >
> > -Evan