On Fri, Nov 27, 2020 at 01:09:41PM +0000, Qais Yousef wrote: > On 11/24/20 15:50, Will Deacon wrote: > > When confronted with a mixture of CPUs, some of which support 32-bit > > Confronted made me laugh, well chosen word! :D > > For some reason made me think of this :p > > https://www.youtube.com/watch?v=NJbXPzSPzxc&t=1m33s I think it just about sums it up! > > applications and others which don't, we quite sensibly treat the system > > as 64-bit only for userspace and prevent execve() of 32-bit binaries. > > > > Unfortunately, some crazy folks have decided to build systems like this > > with the intention of running 32-bit applications, so relax our > > sanitisation logic to continue to advertise 32-bit support to userspace > > on these systems and track the real 32-bit capable cores in a cpumask > > instead. For now, the default behaviour remains but will be tied to > > a command-line option in a later patch. > > > > Signed-off-by: Will Deacon <will@xxxxxxxxxx> > > --- > > arch/arm64/include/asm/cpucaps.h | 2 +- > > arch/arm64/include/asm/cpufeature.h | 8 ++- > > arch/arm64/kernel/cpufeature.c | 106 ++++++++++++++++++++++++++-- > > 3 files changed, 107 insertions(+), 9 deletions(-) > > > > diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h > > index e7d98997c09c..e6f0eb4643a0 100644 > > --- a/arch/arm64/include/asm/cpucaps.h > > +++ b/arch/arm64/include/asm/cpucaps.h > > @@ -20,7 +20,7 @@ > > #define ARM64_ALT_PAN_NOT_UAO 10 > > #define ARM64_HAS_VIRT_HOST_EXTN 11 > > #define ARM64_WORKAROUND_CAVIUM_27456 12 > > -#define ARM64_HAS_32BIT_EL0 13 > > +#define ARM64_HAS_32BIT_EL0_DO_NOT_USE 13 > > nit: would UNUSED be better here? Worth adding a comment as to why too? UNUSED sounds like you could delete it, but I'll add a comment. > > #define ARM64_HARDEN_EL2_VECTORS 14 > > #define ARM64_HAS_CNP 15 > > #define ARM64_HAS_NO_FPSIMD 16 > > [...] > > > +static bool has_32bit_el0(const struct arm64_cpu_capabilities *entry, int scope) > > +{ > > + if (!has_cpuid_feature(entry, scope)) > > + return allow_mismatched_32bit_el0; > > If a user passes the command line by mistake on a 64bit only system, this will > return true. I'll be honest, I'm not entirely sure what the impact is. I get > lost in the features maze. It is nicely encapsulated, but hard to navigate for > the none initiated :-) The thing is, we can't generally detect a 64-bit-only system because a 32-bit-capable CPU could be hotplugged on late. So passing this option just controls what the behaviour is at the point that the 32-bit-capable CPU appears. If one doesn't appear, then there won't be a difference. Will
