[RFC PATCH 0/1] arm64: Fix /proc/cpuinfo

From: Mark Rutland
Date: Fri Oct 24 2014 - 09:57:38 EST

Currently, the arm64 /proc/cpuinfo format differs from that of arm, in a
manner which prevents some otherwise portable applications from
functioning as expected. Specifically, the "Features" line describes the
64-bit hwcaps exclusive of the 32-bit hwcaps, which causes issues for
certain applications which attempt to parse /proc/cpuinfo to detect
features rather than directly using the hwcaps exposed via auxval.

Additionally, the arm64 /proc/cpuinfo format only provides identifying
information for a single CPU (unlike 32-bit), which is problematic for
systems with heterogeneous CPUs (i.e. big.LITTLE).

This patch attempts to solve both issues. I believe the contentious part
is what to do with the Features line, and for that there are a number of
possibilities:

[a] Only print the 64-bit hwcaps

This would match our current behaviour. However certain 32-bit
applications will not detect CPU features correctly, and could fail
to launch. The appropriate hwcaps are available in auxval, but this
will not be of help to existing binaries.

[b] Append the 64-bit and 32-bit hwcaps

This would allow for a consistent format. However, some
human-readable hwcap names have been reused for analogous
instruction set features (e.g. "aes") despite 32-bit and 64-bit
instruction set support being largely unrelated per the
architecture. This could lead to applications mis-detecting
instruction set support on some CPUs in future, and may be
misleading to a casual reader.

[c] Print different hwcaps for compat tasks

This would allow for 32-bit and 64-bit applications to function
correctly. Having the format differ depending on the instruction set
of the application reading /proc/cpuinfo may be misleading in some
cases (e.g. a human using a 32-bit cat to read /proc/cpuinfo on a
64-bit system).

[d] Print different hwcaps dependent on the personality.

This would allow for 32-bit and 64-bit applications to function
correctly, but for some 32-bit applications the personality would
need to be set explicitly by the user.

This patch takes approach d, aligning with what we do for
COMPART_UTS_NAME and COMPAT_ELF_PLATFORM function. Below are sample
output on a 32-bit platform and a 64-bit platform before and after this
patch (with and without LINUX32 personality).

Does it sound reasonable for the /proc/cpuinfo format to vary depending
on the task or personality?

Are there applications for which any of these strategies will not work?

Thanks,
Mark.

[1] arm, v3.17, Versatile Express A15x2 A7x3 coretile
---->8----
processor : 0
model name : ARMv7 Processor rev 1 (v7l)
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt vfpd32 lpae evtstrm
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x2
CPU part : 0xc0f
CPU revision : 1

processor : 1
model name : ARMv7 Processor rev 1 (v7l)
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt vfpd32 lpae evtstrm
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x2
CPU part : 0xc0f
CPU revision : 1

processor : 2
model name : ARMv7 Processor rev 1 (v7l)
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt vfpd32 lpae evtstrm
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x0
CPU part : 0xc07
CPU revision : 1

processor : 3
model name : ARMv7 Processor rev 1 (v7l)
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt vfpd32 lpae evtstrm
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x0
CPU part : 0xc07
CPU revision : 1

processor : 4
model name : ARMv7 Processor rev 1 (v7l)
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt vfpd32 lpae evtstrm
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x0
CPU part : 0xc07
CPU revision : 1

Hardware : ARM-Versatile Express
Revision : 0000
Serial : 0000000000000000
----8<----

[2] arm64, v3.17, Juno platform
---->8----
Processor : AArch64 Processor rev 0 (aarch64)
processor : 0
processor : 1
processor : 2
processor : 3
processor : 4
processor : 5
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: AArch64
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 0

Hardware : Juno
----8<----

[3] arm64, v3.17 + this patch, Juno platform
---->8----
processor : 0
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 0

processor : 1
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 0

processor : 2
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 0

processor : 3
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 0

processor : 4
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd07
CPU revision : 0

processor : 5
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd07
CPU revision : 0
----8<----

[4] arm64, v3.17 + this patch, Juno platform, LINUX32 personality
---->8----
processor : 0
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 0

processor : 1
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 0

processor : 2
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 0

processor : 3
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 0

processor : 4
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd07
CPU revision : 0

processor : 5
Features : half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd07
CPU revision : 0
----8<----

Mark Rutland (1):
arm64: Fix up /proc/cpuinfo

arch/arm64/kernel/setup.c | 96 +++++++++++++++++++++++++++++++++++------------
1 file changed, 72 insertions(+), 24 deletions(-)

--
1.9.1

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