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From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Thomas Gleixner <tglx@linutronix.de>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	arnd@arndb.de, vincenzo.frascino@arm.com, luto@kernel.org
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org,
	x86@kernel.org
Subject: Re: [RFC PATCH v3 08/12] lib: vdso: allow arches to provide vdso data pointer
Date: Wed, 15 Jan 2020 07:15:44 +0100
Message-ID: <d2de3211-9d7c-513e-fe0f-8bdce623fb65@c-s.fr> (raw)
In-Reply-To: <87ftghbpuu.fsf@nanos.tec.linutronix.de>



Le 15/01/2020 à 00:06, Thomas Gleixner a écrit :
> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>>   
>>   static __maybe_unused int
>> +#ifdef VDSO_GETS_VD_PTR_FROM_ARCH
>> +__cvdso_clock_gettime_common(const struct vdso_data *vd, clockid_t clock,
>> +		      struct __kernel_timespec *ts)
>> +{
>> +#else
>>   __cvdso_clock_gettime_common(clockid_t clock, struct __kernel_timespec *ts)
>>   {
>>   	const struct vdso_data *vd = __arch_get_vdso_data();
>> +#endif
>>   	u32 msk;
> 
> If we do that, then there is no point in propagating this to the inner
> functions. It's perfectly fine to have this distinction at the outermost
> level.

In v2, I did it at the arch level (see 
https://patchwork.ozlabs.org/patch/1214983/). Andy was concerned about 
it being suboptimal for arches which (unlike powerpc) have PC related 
data addressing mode.

Wouldn't it be the same issue if doing it at the outermost level of 
generic VDSO ?

> 
> As a related question, I noticed that you keep all that ASM voodoo in
> the PPC specific code which provides the actual entry points. Is that
> ASM code really still necessary? All current users of the generic VDSO
> just do something like:
> 
> int __vdso_clock_gettime(clockid_t clock, struct __kernel_timespec *ts)
> {
>          return __cvdso_clock_gettime(clock, ts);
> }
> 
> in the architecture code. Is there a reason why this can't work on PPC?

The problem with powerpc is that VDSO functions have to (just like 
system calls) set the SO bit in CR register in case of error, or clear 
it if no error. There is no way to do that from the C function, because 
there is no way to tell GCC to not play up with CR register on function 
return.

Refer discussion at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92769

Christophe

  reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-13 17:08 [RFC PATCH v3 00/12] powerpc: switch VDSO to C implementation Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 01/12] powerpc/64: Don't provide time functions in compat VDSO32 Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 02/12] powerpc/vdso: Switch VDSO to generic C implementation Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 03/12] lib: vdso: mark __cvdso_clock_getres() as static Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 04/12] lib: vdso: inline do_hres() and do_coarse() Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 05/12] lib: vdso: Avoid duplication in __cvdso_clock_getres() Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 06/12] lib: vdso: __iter_div_u64_rem() is suboptimal for 32 bit time Christophe Leroy
2020-01-14 11:31   ` Thomas Gleixner
2020-01-13 17:08 ` [RFC PATCH v3 07/12] powerpc/vdso: simplify __get_datapage() Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 08/12] lib: vdso: allow arches to provide vdso data pointer Christophe Leroy
2020-01-14 23:06   ` Thomas Gleixner
2020-01-15  6:15     ` Christophe Leroy [this message]
2020-01-16  9:16       ` Christophe Leroy
2020-01-16 10:35         ` Thomas Gleixner
2020-01-16 20:22           ` Andy Lutomirski
2020-01-13 17:08 ` [RFC PATCH v3 09/12] powerpc/vdso: provide inline alternative to __get_datapage() Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 10/12] powerpc/vdso: provide vdso data pointer from the ASM caller Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 11/12] lib: vdso: split clock verification out of __arch_get_hw_counter() Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 12/12] powerpc/vdso: provide __arch_is_hw_counter_valid() Christophe Leroy

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