Re: [PATCH] riscv: add asm/unistd.h UAPI header

From: Arnd Bergmann <arnd@arndb.de>
To: Palmer Dabbelt <palmer@sifive.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>,
	David Abdurachmanov <david.abdurachmanov@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>,
	linux-riscv@lists.infradead.org,
	Guenter Roeck <linux@roeck-us.net>
Subject: Re: [PATCH] riscv: add asm/unistd.h UAPI header
Date: Thu, 8 Nov 2018 11:30:02 +0100	[thread overview]
Message-ID: <CAK8P3a2468O=MVXVaZ8_Vq6DuOYV2iwR98upqpWOH67z9Gh-6Q@mail.gmail.com> (raw)
Message-ID: <20181108103002.NjR-CtO4ETKMIxznyGKjTjTGwDbAeCLsRxMAvZdAtFE@z> (raw)
In-Reply-To: <mhng-23f94805-b961-4e68-a4c1-7d79b60bd925@palmer-si-x1c4>

On Thu, Nov 8, 2018 at 3:10 AM Palmer Dabbelt <palmer@sifive.com> wrote:
>
> On Wed, 07 Nov 2018 13:09:39 PST (-0800), Arnd Bergmann wrote:
> > On Wed, Nov 7, 2018 at 7:30 PM David Abdurachmanov
> > <david.abdurachmanov@gmail.com> wrote:
> >> On Wed, Nov 7, 2018 at 1:08 AM Palmer Dabbelt <palmer@sifive.com> wrote:
> >> > On Mon, 05 Nov 2018 12:56:15 PST (-0800), Arnd Bergmann wrote:
> >
> >> > The target is still the next glibc release (Feb 1st) for a stable RV32I ABI.
> >> > That's progressing well, with one last blocking issue related to some of our
> >> > floating-point emulation routines before we can submit the port.  This should
> >> > give us ample time to line up the ABIs correctly so everything works.
> >> >
> >> > So I think the correct answer here is to drop __ARCH_WANT_STAT64 from RISC-V.
> >> >
> >>
> >> Then if you agree I could do and send v2:
> >>
> >> +#ifdef __LP64__
> >> +#define __ARCH_WANT_NEW_STAT
> >> +#endif /* __LP64__ */
> >
> > Looks good to me.
>
> This is a bit pedantic, but I'm not sure what the right answer is here:
> "-march=rv64gc -mabi=ilp32d" will not define __LP64__, but will define
> "__riscv_xlen == 64".  I actually don't know enough about how an rv64gc/ilp32d
> ABI would work to answer this: would we have "long long" all over our syscalls?
>
> Probably not worth worrying about for now, as we'll have to go audit all of
> these if we ever end up with an ilp32 ABI.  So just go for it and we'll throw
> this on the pile to deal with later :)

Short answer: it doesn't matter because an ilp32d ABI would use neither
newstat nor stat64, it would only need statx().

Long answer: We've gone through multiple iterations on the question.
x86 uses long long in syscall interfaces and tries to reuse the native
64-bit syscalls as much as possible. This turned out to cause endless
problems, so for the (never merged but still kept around as a patchset)
arm64 ABI, we went the opposite way, and made the syscalls use the
same ABI as the arm32 mode.

From the experience with both of the above, I'd say if you end up
having to do it, use the same method as arm64, but try to resist
doing it at all. Unlike arm64 and x86-64, there is no inherent benefit
to using the 64-bit instruction set (doubled register number etc),
so compared to the normal lp64 ABI you only gain a little dcache
space for the smaller pointers at the cost of a smaller address
space. For you as a maintainer however, the cost of supporting this
mode is that you are stuck with three user space ABIs instead of
just two (normal 32-bit and 64-bit).
If anyone really wants to run 32-bit code, they need a CPU that
allows switching modes.

       Arnd

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