From: Adhemerval Zanella <adhemerval.zanella@linaro.org>
To: Rich Felker <dalias@libc.org>, Nicholas Piggin <npiggin@gmail.com>
Cc: libc-alpha@sourceware.org, libc-dev@lists.llvm.org,
linuxppc-dev@lists.ozlabs.org, musl@lists.openwall.com
Subject: Re: [musl] Powerpc Linux 'scv' system call ABI proposal take 2
Date: Thu, 23 Apr 2020 09:13:57 -0300 [thread overview]
Message-ID: <a972a0e4-25bd-9cf5-75ec-2eb993ef1b6a@linaro.org> (raw)
In-Reply-To: <20200423023642.GP11469@brightrain.aerifal.cx>
On 22/04/2020 23:36, Rich Felker wrote:
> On Wed, Apr 22, 2020 at 04:18:36PM +1000, Nicholas Piggin wrote:
>> Yeah I had a bit of a play around with musl (which is very nice code I
>> must say). The powerpc64 syscall asm is missing ctr clobber by the way.
>> Fortunately adding it doesn't change code generation for me, but it
>> should be fixed. glibc had the same bug at one point I think (probably
>> due to syscall ABI documentation not existing -- something now lives in
>> linux/Documentation/powerpc/syscall64-abi.rst).
>
> Do you know anywhere I can read about the ctr issue, possibly the
> relevant glibc bug report? I'm not particularly familiar with ppc
> register file (at least I have to refamiliarize myself every time I
> work on this stuff) so it'd be nice to understand what's
> potentially-wrong now.
My understanding is the ctr issue only happens for vDSO calls where it
fallback to a syscall in case an error (invalid argument, etc. and
assuming if vDSO does not fallback to a syscall it always succeed).
This makes the vDSO call on powerpc to have same same ABI constraint
as a syscall, where it clobbers CR0.
On glibc we handle by simulating a function call and analysing the CR0
result:
__asm__ __volatile__
("mtctr %0\n\t"
"bctrl\n\t"
"mfcr %0\n\t"
"0:"
: "+r" (r0), "+r" (r3), "+r" (r4), "+r" (r5), "+r" (r6),
"+r" (r7), "+r" (r8)
: : "r9", "r10", "r11", "r12", "cr0", "ctr", "lr", "memory");
__asm__ __volatile__ ("" : "=r" (rval) : "r" (r3));
On musl you don't have this issue because it does not enable vDSO
support on powerpc. And if it eventually does it with the VDSO_*
macros the only issue I see is on when vDSO fallbacks to the syscall
and it also fails (the return code won't be negated since on musl it
uses a default C function pointer issue which does not model the CR0
kernel abi).
So I think the extra ctr constraint on glibc powerpc syscall code is
not really required. I think I have some patches to optimize this a
bit based on previous discussions.
next prev parent reply other threads:[~2020-04-23 12:14 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-15 21:45 Nicholas Piggin
2020-04-15 22:55 ` [musl] " Rich Felker
2020-04-16 0:16 ` Nicholas Piggin
2020-04-16 0:48 ` Rich Felker
2020-04-16 2:24 ` Nicholas Piggin
2020-04-16 2:35 ` Rich Felker
2020-04-16 2:53 ` Nicholas Piggin
2020-04-16 3:03 ` Rich Felker
2020-04-16 3:41 ` Nicholas Piggin
2020-04-16 20:18 ` Florian Weimer
2020-04-16 9:58 ` Szabolcs Nagy
2020-04-20 0:27 ` Nicholas Piggin
2020-04-20 1:29 ` Rich Felker
2020-04-20 2:08 ` Nicholas Piggin
2020-04-20 21:17 ` Szabolcs Nagy
2020-04-21 9:57 ` Florian Weimer
2020-04-16 15:21 ` Jeffrey Walton
2020-04-16 15:40 ` Rich Felker
2020-04-16 4:48 ` Florian Weimer
2020-04-16 15:35 ` Rich Felker
2020-04-16 16:42 ` Florian Weimer
2020-04-16 16:52 ` Rich Felker
2020-04-16 18:12 ` Florian Weimer
2020-04-16 23:02 ` Segher Boessenkool
2020-04-17 0:34 ` Rich Felker
2020-04-17 1:48 ` Segher Boessenkool
2020-04-17 8:34 ` Florian Weimer
2020-04-16 14:16 ` Adhemerval Zanella
2020-04-16 15:37 ` Rich Felker
2020-04-16 17:50 ` Adhemerval Zanella
2020-04-16 17:59 ` Rich Felker
2020-04-16 18:18 ` Adhemerval Zanella
2020-04-16 18:31 ` Rich Felker
2020-04-16 18:44 ` Rich Felker
2020-04-16 18:52 ` Adhemerval Zanella
2020-04-20 0:46 ` Nicholas Piggin
2020-04-20 1:10 ` Nicholas Piggin
2020-04-20 1:34 ` Rich Felker
2020-04-20 2:32 ` Nicholas Piggin
2020-04-20 4:09 ` Rich Felker
2020-04-20 4:31 ` Nicholas Piggin
2020-04-20 17:27 ` Rich Felker
2020-04-22 6:18 ` Nicholas Piggin
2020-04-22 6:29 ` Nicholas Piggin
2020-04-23 2:36 ` Rich Felker
2020-04-23 12:13 ` Adhemerval Zanella [this message]
2020-04-23 16:18 ` Rich Felker
2020-04-23 16:35 ` Adhemerval Zanella
2020-04-23 16:43 ` Rich Felker
2020-04-23 17:15 ` Adhemerval Zanella
2020-04-23 17:42 ` Rich Felker
2020-04-25 3:40 ` Nicholas Piggin
2020-04-25 4:52 ` Rich Felker
2020-04-25 3:30 ` Nicholas Piggin
2020-04-21 12:28 ` David Laight
2020-04-21 14:39 ` Rich Felker
2020-04-21 15:00 ` Adhemerval Zanella
2020-04-21 15:31 ` David Laight
2020-04-22 6:54 ` Nicholas Piggin
2020-04-22 7:15 ` [musl] " Florian Weimer
2020-04-22 7:31 ` Nicholas Piggin
2020-04-22 8:11 ` Florian Weimer
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