On Fri, Jan 16, 2015 at 04:44:34PM +0000, Will Deacon wrote: > On Fri, Jan 16, 2015 at 04:32:54PM +0000, Mark Rutland wrote: > > On Fri, Jan 16, 2015 at 04:07:30PM +0000, Will Deacon wrote: > > > On Thu, Jan 15, 2015 at 12:36:05PM +0000, Suzuki K. Poulose wrote: > > > > From: "Suzuki K. Poulose" <suzuki.poul...@arm.com> > > > > > > > > As of now each insn_emulation has a cpu hotplug notifier that > > > > enables/disables the CPU feature bit for the functionality. This > > > > patch re-arranges the code, such that there is only one notifier > > > > that runs through the list of registered emulation hooks and runs > > > > their corresponding set_hw_mode. > > > > > > > > We do nothing when a CPU is dying as we will set the appropriate bits > > > > as it comes back online based on the state of the hooks. > > > > > > > > Signed-off-by: Suzuki K. Poulose <suzuki.poul...@arm.com> > > > > Signed-off-by: Mark Rutland <mark.rutl...@arm.com> > > > > --- > > > > Documentation/arm64/legacy_instructions.txt | 4 + > > > > arch/arm64/include/asm/cpufeature.h | 2 + > > > > arch/arm64/kernel/armv8_deprecated.c | 113 > > > > +++++++++++++++------------ > > > > 3 files changed, 69 insertions(+), 50 deletions(-) > > > > > > > > diff --git a/Documentation/arm64/legacy_instructions.txt > > > > b/Documentation/arm64/legacy_instructions.txt > > > > index a3b3da2..0a4dc26 100644 > > > > --- a/Documentation/arm64/legacy_instructions.txt > > > > +++ b/Documentation/arm64/legacy_instructions.txt > > > > @@ -27,6 +27,10 @@ behaviours and the corresponding values of the > > > > sysctl nodes - > > > > instructions. Using hardware execution generally provides better > > > > performance, but at the loss of ability to gather runtime statistics > > > > about the use of the deprecated instructions. > > > > + Note: Emulation of a deprecated instruction depends on the > > > > availability > > > > + of the feature on all the active CPUs. In case of CPU hotplug, if a > > > > new > > > > + CPU doesn't support a feature, it could result in the abortion of the > > > > + hotplug operation. > > > > > > Is this true? We should be able to *emulate* the instruction anywhere, > > > it's the "hardware execution" setting that needs CPU support. > > > > Not quite. In ARMv8 there are three possible cases for endianness > > support: > > > > (a) Mixed endian support at all levels > > ID_AA64MMFR0_EL1.BigEnd == 0b0001 > > ID_AA64MMFR0_EL1.BigEnd0 == 0b0000 (RES0, invalid) > > > > (b) Mixed endian support at EL0 only > > ID_AA64MMFR0_EL1.BigEnd == 0b0000 > > ID_AA64MMFR0_EL1.BigEnd0 == 0b0001 > > SCTLR_EL1.EE has a fixed value > > > > (c) No mixed endian support at all > > ID_AA64MMFR0_EL1.BigEnd == 0b0000 > > ID_AA64MMFR0_EL1.BigEnd0 == 0b0000 > > SCTLR_EL1.{EE,E0E} have fixed values > > > > We can emulate setend in cases (a) and (b), but not (c) unless we > > trapped every single instruction that could access memory. I don't think > > that's feasible. > > > > Also fun in case (c) is that the kernel may not be able to run on all > > CPUs (consider a BE kernel where some CPUs are fixed to LE at EL1). > > > > I hope no-one builds a system where CPUs are mismatched w.r.t. > > endianness support. > > Agreed, the problem here is all down to the wording. > Documentation/arm64/legacy_instructions.txt isn't specific to SETEND > emulation and saying "Emulation of a deprecated instruction depends on the > availability of the feature on all the active CPUs." as a blanket statement > doesn't make a lot of sense in isolation. It's certainly not the case for > SWP and CP15 barriers, for example. > > With SETEND, we have requirements on mixed-endian support for the emulation. > Fine, but let's not muddy the waters around the general CPU requirements > for deprecated instruction emulation.
Sure. I was only contending the "We should be able to *emulate* the instruction anywhere" part of your statement. We should probably add _a_ note as part of the patch introducing SETEND emulation, but not the one above, and we can drop it from this patch. Mark. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/