[RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation.

From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	nathanl@linux.ibm.com, arnd@arndb.de, tglx@linutronix.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: [RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation.
Date: Thu, 16 Jan 2020 17:58:24 +0000 (UTC)
Message-ID: <cover.1579196675.git.christophe.leroy@c-s.fr> (raw)

This is a fourth tentative to switch powerpc VDSO to generic C implementation.

This version should work on PPC64 (untested). VDSO32 for PPC64 is
impossible to build and has been de-activated, because the powerpc
ASM header files for C are not prepared to build 32 bits code with CONFIG_PPC64.

powerpc is a bit special for VDSO as well as system calls in the
way that it requires setting CR SO bit which cannot be done in C.
Therefore, entry/exit and fallback need to be performed in ASM.

Note that on previous patches, a buggy version of vdsotest was used which was
underestimating the time in gettimeofday compared to clock-get... functions.

This series applies on a merge of powerpc/merge and tip/timers/core branches,
series "lib/vdso: Bugfix and consolidation"
(https://lore.kernel.org/patchwork/project/lkml/list/?series=425784)
applied after the above merge.

On a powerpc8xx, with current powerpc/32 ASM VDSO:

gettimeofday:    vdso: 907 nsec/call
clock-getres-realtime:    vdso: 484 nsec/call
clock-gettime-realtime:    vdso: 899 nsec/call

The first patch adds VDSO generic C support without any changes to common code.
Performance is as follows:

gettimeofday:    vdso: 1211 nsec/call
clock-getres-realtime:    vdso: 722 nsec/call
clock-gettime-realtime:    vdso: 1216 nsec/call

Then a few changes in the common code have allowed performance improvement. At
the end of the series we have:

gettimeofday:    vdso: 974 nsec/call
clock-getres-realtime:    vdso: 545 nsec/call
clock-gettime-realtime:    vdso: 941 nsec/call

The final result is rather close to pure ASM VDSO:
* 7% more on gettimeofday (9 cycles)
* 5% more on clock-gettime-realtime (6 cycles)
* 12% more on clock-getres-realtime (8 cycles)

Due to the unavoidable ASM trampoline, we won't get much closer but that should be
acceptable for a port from ASM to a generic C code (here, 1 cycle is about 7,5 ns)

Christophe Leroy (11):
  powerpc/64: Don't provide time functions in compat VDSO32
  powerpc/vdso: Switch VDSO to generic C implementation.
  lib: vdso: only read hrtimer_res when needed in __cvdso_clock_getres()
  powerpc/vdso: simplify __get_datapage()
  lib: vdso: allow arches to provide vdso data pointer
  powerpc/vdso: provide inline alternative to __get_datapage()
  powerpc/vdso: provide vdso data pointer from the ASM caller.
  lib: vdso: allow fixed clock mode
  powerpc/vdso: override __arch_vdso_capable()
  lib: vdso: Allow arches to override the ns shift operation
  powerpc/32: provide vdso_shift_ns()

 arch/powerpc/Kconfig                         |   2 +
 arch/powerpc/include/asm/clocksource.h       |   6 +
 arch/powerpc/include/asm/vdso/gettimeofday.h | 117 ++++++++++++
 arch/powerpc/include/asm/vdso/vsyscall.h     |  25 +++
 arch/powerpc/include/asm/vdso_datapage.h     |  52 +++---
 arch/powerpc/kernel/asm-offsets.c            |  46 +----
 arch/powerpc/kernel/time.c                   |  91 +---------
 arch/powerpc/kernel/vdso.c                   |  58 ++----
 arch/powerpc/kernel/vdso32/Makefile          |  27 ++-
 arch/powerpc/kernel/vdso32/datapage.S        |  10 +-
 arch/powerpc/kernel/vdso32/gettimeofday.S    | 258 ++++-----------------------
 arch/powerpc/kernel/vdso32/vdso32.lds.S      |   9 +-
 arch/powerpc/kernel/vdso32/vgettimeofday.c   |  29 +++
 arch/powerpc/kernel/vdso64/Makefile          |  23 ++-
 arch/powerpc/kernel/vdso64/datapage.S        |  13 +-
 arch/powerpc/kernel/vdso64/gettimeofday.S    | 257 ++++----------------------
 arch/powerpc/kernel/vdso64/vdso64.lds.S      |   7 +-
 arch/powerpc/kernel/vdso64/vgettimeofday.c   |  29 +++
 lib/vdso/gettimeofday.c                      | 107 ++++++++---
 19 files changed, 457 insertions(+), 709 deletions(-)
 create mode 100644 arch/powerpc/include/asm/clocksource.h
 create mode 100644 arch/powerpc/include/asm/vdso/gettimeofday.h
 create mode 100644 arch/powerpc/include/asm/vdso/vsyscall.h
 create mode 100644 arch/powerpc/kernel/vdso32/vgettimeofday.c
 create mode 100644 arch/powerpc/kernel/vdso64/vgettimeofday.c

-- 
2.13.3