On Sep 2, 2015, at 12:33 AM, Mark Rutland [off-list ref] wrote:

Hi,

quoted

On Sat, Aug 29, 2015 at 07:46:22PM +0100, Andrew Pinski wrote:
It is useful to pass down MIDR register down to userland if all of
the online cores are all the same type.  This adds AT_ARM64_MIDR
aux vector type and passes down the midr system register.

This is alternative to MIDR_EL1 part of
http://lists.infradead.org/pipermail/linux-arm-kernel/2015-July/358995.html.
It allows for faster access to midr_el1 than going through a trap and
does not exist if the set of cores are not the same.

I'm not sure I follow the rationale. If speed is important the
application can cache the value the first time it reads it with a trap.

It is also about compatibility also. Exposing the register is not backwards compatible but using the aux vector is. 

This also means that the behaviour is different across homogeneous and
heterogeneous systems.

quoted

Changes from v1:
Forgot to include the auxvec.h part.

Signed-off-by: Andrew Pinski <redacted>
---
arch/arm64/include/asm/cpu.h         |    1 +
arch/arm64/include/asm/elf.h         |    6 ++++++
arch/arm64/include/uapi/asm/auxvec.h |    3 +++
arch/arm64/kernel/cpuinfo.c          |   22 ++++++++++++++++++++++
4 files changed, 32 insertions(+)

diff --git a/arch/arm64/include/asm/cpu.h b/arch/arm64/include/asm/cpu.h
index 8e797b2..fab0aa1 100644
--- a/arch/arm64/include/asm/cpu.h
+++ b/arch/arm64/include/asm/cpu.h

@@ -62,5 +62,6 @@ DECLARE_PER_CPU(struct cpuinfo_arm64, cpu_data);

void cpuinfo_store_cpu(void);
void __init cpuinfo_store_boot_cpu(void);
+u32 get_arm64_midr(void);

#endif /* __ASM_CPU_H */

diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index faad6df..d3549de 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h

@@ -17,6 +17,7 @@

#define __ASM_ELF_H

#include <asm/hwcap.h>
+#include <asm/cpu.h>

/*
 * ELF register definitions..

@@ -138,8 +139,13 @@ typedef struct user_fpsimd_state elf_fpregset_t;

#define ARCH_DLINFO                            \
do {                                    \
+    u32 midr;                            \
+                                    \
   NEW_AUX_ENT(AT_SYSINFO_EHDR,                    \
           (elf_addr_t)current->mm->context.vdso);        \
+    midr = get_arm64_midr();                    \
+    if (midr != 0)                            \
+        NEW_AUX_ENT(AT_ARM64_MIDR, (elf_addr_t)midr);        \
} while (0)

#define ARCH_HAS_SETUP_ADDITIONAL_PAGES

diff --git a/arch/arm64/include/uapi/asm/auxvec.h b/arch/arm64/include/uapi/asm/auxvec.h
index 22d6d88..dc55c56 100644
--- a/arch/arm64/include/uapi/asm/auxvec.h
+++ b/arch/arm64/include/uapi/asm/auxvec.h

@@ -19,4 +19,7 @@

/* vDSO location */
#define AT_SYSINFO_EHDR    33

+/* Machine IDenfier Register (MDIR). */
+#define AT_ARM64_MIDR 38
+
#endif

diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
index 75d5a86..b14c87d 100644
--- a/arch/arm64/kernel/cpuinfo.c
+++ b/arch/arm64/kernel/cpuinfo.c

@@ -254,3 +254,25 @@ void __init cpuinfo_store_boot_cpu(void)

   boot_cpu_data = *info;

}
+
+u32 get_arm64_midr(void)
+{
+    int i;
+    u32 midr = 0;
+
+    for_each_online_cpu(i) {
+        struct cpuinfo_arm64 *cpuinfo = &per_cpu(cpu_data, i);
+        u32 oldmidr = midr;
+
+        midr = cpuinfo->reg_midr;
+        /*
+         * If there are cpus which have a different
+         * midr just return 0.
+         */
+        if (oldmidr && oldmidr != midr)
+            return 0;
+    }
+
+    return midr;
+}

If I have a big.LITTLE system where all the big CPUs are currently
offline, this will leave the MIDR the little CPUs in the auxvec.
However, at any point after this has run, I could hotplug the big CPUs
on and the little CPUs off, leaving this reporting a MIDR that
represents none of the online CPUs.

Given big.LITTLE and the potential for physical/dynamic hotplug (where
we won't know all the MIDRs in advance), I don't think that we can
generally expose a common MIDR in this fashion, and I don't think that
we should give the impression that we can.

This is standard issue with hot plug and big.little. Really big.little is a design flaw but I am not going into that here. 

I think that the only things we can do are expose the MIDR for CPU the
code is currently executing on (as Suzuki's patches do), and/or expose
all the MIDRs for currently online CPUs (as Steve's [1] patch does).
Anything else leaves us trying to provide semantics that we cannot
guarantee.

Except they are not backwards compatible which means nobody in their right mind would use the register to get the midr that way. I am sorry but having a newer version of glibc working on a year old kernel is not going to fly. 

Thanks,
Andrew

Thanks,
Mark.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-July/359127.html