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

[Palmer Dabbelt <palmer@rivosinc.com>]

On Thu, 15 Feb 2024 09:00:03 PST (-0800), enh@google.com wrote:
> 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.

So you're already committed to that ABI?

> (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