[PATCH 0/4] arm64: advertise availability of CRC and crypto instructions

[Russell King - ARM Linux]

On Wed, Dec 18, 2013 at 10:50:38AM +0100, Ard Biesheuvel wrote:
> On 17 December 2013 13:25, Catalin Marinas <catalin.marinas at arm.com> wrote:
> > On Mon, Dec 16, 2013 at 09:04:34PM +0000, Ard Biesheuvel wrote:
> >> This series is an expansion of the patch posted by Steve Capper about 6 weeks
> >> ago that allocates hwcaps bits for CRC and Crypto Extensions instructions so
> >> userland can discover whether the current CPU has any of those capabilities.
> >>
> >> Patch #1 is a cleanup patch for read_cpuid(), which allowed me to skip adding
> >> yet another #define to asm/cputype.h (for ID_ISAR5_EL1)
> >>
> >> Patch #2 is Steve's original patch, but slightly tweaked because hwcaps bit 2
> >> has been allocated for something else in the mean time.
> >>
> >> Patch #3 allocates the capability bits in the arch/arm tree. This is necessary
> >> because 32-bit ARM binaries can execute both under ARM and under arm64 kernels,
> >> so there should be agreement about the meaning of feature bits, even if those
> >> features don't actually exist on systems covered by the arch/arm tree.
> >>
> >> @Russell: if this looks ok to you, could you please indicate whether you prefer
> >> to take this patch separately, or ack it and let it be merged as part of the
> >> series.
> >>
> >> Patch #4 advertises the CRC and Crypto Extensions to 32-bit binaries running
> >> under an arm64 kernel.
> >
> > The series look fine to me. I'm happy to take all the patches if Russell
> > Acks the arm one (or it can send it via the patch system).
> >
> 
> Hello Russell,
> 
> Care to share your take on this? I imagine new hwcaps bits take a
> while to percolate and make their way into a stable glibc, so I would
> like to have these changes in sooner rather than later, and 3.14 seems
> feasible.

I'm not all that happy as it gobbles up a load of bits in the hwcap that
will never be set for ARM, and we only have 32 of them (limited by the
size of elf_addr_t).  On ARM64, it's less of a problem because the hwcap
is 64-bit there.

If we allocate the ARM64 private never-will-appear-on-ARM hwcaps in bit
32 and above, they'll be hidden from 32-bit stuff.  Hopefully, glibc
doesn't concatenate the HWCAP and HWCAP2 fields though - someone should
check that.

Since the bits in the ARM64 hwcap are different from the ARM32 hwcap, I
don't see any point in defining them for ARM32 - userspace needs to make
the definition conditional anyway, and can't interpret the bits as-is
because ARM64 already omits many of the ARM32 ones.