Thomas Gleixner 9d90b93bf3 lib/vdso: Make delta calculation work correctly ... The x86 vdso implementation on which the generic vdso library is based on has subtle (unfortunately undocumented) twists: 1) The code assumes that the clocksource mask is U64_MAX which means that no bits are masked. Which is true for any valid x86 VDSO clocksource. Stupidly it still did the mask operation for no reason and at the wrong place right after reading the clocksource. 2) It contains a sanity check to catch the case where slightly unsynchronized TSC values can be observed which would cause the delta calculation to make a huge jump. It therefore checks whether the current TSC value is larger than the value on which the current conversion is based on. If it's not larger the base value is used to prevent time jumps. #1 Is not only stupid for the X86 case because it does the masking for no reason it is also completely wrong for clocksources with a smaller mask which can legitimately wrap around during a conversion period. The core timekeeping code does it correct by applying the mask after the delta calculation: (now - base) & mask #2 is equally broken for clocksources which have smaller masks and can wrap around during a conversion period because there the now > base check is just wrong and causes stale time stamps and time going backwards issues. Unbreak it by: 1) Removing the mask operation from the clocksource read which makes the fallback detection work for all clocksources 2) Replacing the conditional delta calculation with a overrideable inline function. #2 could reuse clocksource_delta() from the timekeeping code but that results in a significant performance hit for the x86 VSDO. The timekeeping core code must have the non optimized version as it has to operate correctly with clocksources which have smaller masks as well to handle the case where TSC is discarded as timekeeper clocksource and replaced by HPET or pmtimer. For the VDSO there is no replacement clocksource. If TSC is unusable the syscall is enforced which does the right thing. To accommodate to the needs of various architectures provide an override-able inline function which defaults to the regular delta calculation with masking: (now - base) & mask Override it for x86 with the non-masking and checking version. This unbreaks the ARM64 syscall fallback operation, allows to use clocksources with arbitrary width and preserves the performance optimization for x86. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com> Cc: linux-arch@vger.kernel.org Cc: LAK <linux-arm-kernel@lists.infradead.org> Cc: linux-mips@vger.kernel.org Cc: linux-kselftest@vger.kernel.org Cc: catalin.marinas@arm.com Cc: Will Deacon <will.deacon@arm.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: linux@armlinux.org.uk Cc: Ralf Baechle <ralf@linux-mips.org> Cc: paul.burton@mips.com Cc: Daniel Lezcano <daniel.lezcano@linaro.org> Cc: salyzyn@android.com Cc: pcc@google.com Cc: shuah@kernel.org Cc: 0x7f454c46@gmail.com Cc: linux@rasmusvillemoes.dk Cc: huw@codeweavers.com Cc: sthotton@marvell.com Cc: andre.przywara@arm.com Cc: Andy Lutomirski <luto@kernel.org> Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1906261159230.32342@nanos.tec.linutronix.de		2019-06-26 14:26:53 +02:00
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gettimeofday.c	lib/vdso: Make delta calculation work correctly	2019-06-26 14:26:53 +02:00
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