From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Thu, 3 Jul 2014 20:00:55 +0100 Subject: Android and compatibility with deprecated armv7 instructions In-Reply-To: <20140703185243.GD32514@n2100.arm.linux.org.uk> References: <20140703150008.GX32514@n2100.arm.linux.org.uk> <20140703171356.GD28175@arm.com> <4840595.cpHbY6hJKL@wuerfel> <20140703183055.GC32514@n2100.arm.linux.org.uk> <20140703183739.GA21086@arm.com> <20140703185243.GD32514@n2100.arm.linux.org.uk> Message-ID: <20140703190055.GA21394@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, Jul 03, 2014 at 07:52:43PM +0100, Russell King - ARM Linux wrote: > On Thu, Jul 03, 2014 at 07:37:39PM +0100, Will Deacon wrote: > > On Thu, Jul 03, 2014 at 07:30:55PM +0100, Russell King - ARM Linux wrote: > > > which produces the correct answer of 5. SWP_EMULATE is off. HWCAP_SWP > > > is set (as it always has been). There's no warning in the kernel > > > message log. > > > > Even more worrying, that swp you just executed probably wasn't atomic. Try > > it with a multithreaded program... > > > > > Now look at the situation on ARMv8. Executing the above suddenly fails > > > with SIGILL. > > > > > > This means that from the developer and user point of view, there has > > > been *no* visible deprecation of this instruction. There's only > > > "it used to work on ARMv7 but doesn't on ARMv8". > > > > > > Yes, SWP_EMULATE was introduced. Great, that has the power to issue > > > warnings, except it doesn't because they're at DEBUG level, which > > > normally gets compiled out. So that doesn't help warn people. > > > > > > Even with SWP_EMULATE enabled and with the debug problem fixed... does > > > it help warn people? Only if you're running on a CPU with virtualisation > > > extensions, because it silently continues to work on CPUs without. > > > > Not *quite* true; SWP_EMULATE=y disables the h/w SWP instruction by > > configuring the sctlr accordingly. So even without virtualisation > > extensions (more specifically, on cores with MP extensions), you would > > still see the warning if it was fixed. > > Well, does anyone have SWP_EMULATE enabled? > > arch/arm/configs/spear13xx_defconfig:# CONFIG_SWP_EMULATE is not set > arch/arm/configs/mvebu_v7_defconfig:# CONFIG_SWP_EMULATE is not set > arch/arm/configs/prima2_defconfig:# CONFIG_SWP_EMULATE is not set > arch/arm/configs/marzen_defconfig:# CONFIG_SWP_EMULATE is not set > arch/arm/configs/shmobile_defconfig:# CONFIG_SWP_EMULATE is not set > arch/arm/configs/koelsch_defconfig:# CONFIG_SWP_EMULATE is not set > arch/arm/configs/vexpress_defconfig:# CONFIG_SWP_EMULATE is not set > > I think not. Even with SWP_EMULATE set, HWCAP_SWP is still > advertised, which you must admit is a very contradictory situation. Yup, it's probably worth trying to address these inconsistencies regardless of the conclusion for the 64-bit kernel. Removing the hwcap and screaming when emulation happens both sound very sensible to me. Will