[PATCH] arm64: cpufeature: Expose the real mpidr value to EL0
Robin Murphy
robin.murphy at arm.com
Tue Sep 12 01:31:43 PDT 2023
On 2023-09-12 04:52, guojinhui.liam wrote:
> In EL0, it can get the register midr's value to distinguish vendor.
> But it won't return real value of the register mpidr by using mrs
> in EL0. The register mpidr's value is useful to obtain the cpu
> topology information.
...except there's no guarantee that the MPIDR value is anything other
than a unique identifier. Proper topology information is already exposed
to userspace[1], as described by ACPI PPTT or Devicetree[2]. Userspace
should be using that.
Not to mention that userspace fundamentally can't guarantee it won't be
migrated at just the wrong point and read the MPIDR of a different CPU
anyway. (This is why the MIDRs and REVIDRs are also reported via sysfs,
such that userspace has a stable and reliable source of information in
case it needs to consider potential errata.)
Thanks,
Robin.
[1] https://www.kernel.org/doc/html/latest/admin-guide/cputopology.html
[2]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/cpu/cpu-topology.txt
> In some scenarios, the task scheduling in userspace can be
> optimized with CPU Die information.
>
> Signed-off-by: guojinhui.liam <guojinhui.liam at bytedance.com>
> ---
> arch/arm64/include/asm/sysreg.h | 3 ---
> arch/arm64/kernel/cpufeature.c | 2 +-
> 2 files changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 38296579a4fd..1885857c8a22 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -901,9 +901,6 @@
> #define SYS_TFSR_EL1_TF0 (UL(1) << SYS_TFSR_EL1_TF0_SHIFT)
> #define SYS_TFSR_EL1_TF1 (UL(1) << SYS_TFSR_EL1_TF1_SHIFT)
>
> -/* Safe value for MPIDR_EL1: Bit31:RES1, Bit30:U:0, Bit24:MT:0 */
> -#define SYS_MPIDR_SAFE_VAL (BIT(31))
> -
> #define TRFCR_ELx_TS_SHIFT 5
> #define TRFCR_ELx_TS_MASK ((0x3UL) << TRFCR_ELx_TS_SHIFT)
> #define TRFCR_ELx_TS_VIRTUAL ((0x1UL) << TRFCR_ELx_TS_SHIFT)
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index b018ae12ff5f..6e18597fdcc3 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -3450,7 +3450,7 @@ static inline int emulate_id_reg(u32 id, u64 *valp)
> *valp = read_cpuid_id();
> break;
> case SYS_MPIDR_EL1:
> - *valp = SYS_MPIDR_SAFE_VAL;
> + *valp = read_cpuid_mpidr();
> break;
> case SYS_REVIDR_EL1:
> /* IMPLEMENTATION DEFINED values are emulated with 0 */
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