Hi,
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.
This also means that the behaviour is different across homogeneous and
heterogeneous systems.
> Changes from v1:
> Forgot to include the auxvec.h part.
>
> Signed-off-by: Andrew Pinski <apin...@cavium.com>
> ---
> 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.
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.
Thanks,
Mark.
[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-July/359127.html
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