Subject: [RFC PATCH] arm64: Expose physical/virtual address bits through cpuinfo

From: [email-masked] (Suzuki K. Poulose)
Date: Thu, 17 Mar 2016 11:31:40 +0000

On 17/03/16 10:08, Kefeng Wang wrote:
> AArch64 support six types Physical Address range, permitted values
> in ID_AA64MMFR0_EL1 are:
> 0000  32 bits, 4GB.
> 0001  36 bits, 64GB.
> 0010  40 bits, 1TB.
> 0011  42 bits, 4TB.
> 0100  44 bits, 16TB.
> 0101  48 bits, 256TB.
> All other values are reserved.
>
> Meanwhile, AArch64 allows 36-bit, 39-bit, 42-bit, 47-bit and 48-bit
> virtual addresses.

See below..

>
> +static inline int id_aa64mmfr0_parange_bits(void)
> +{
> +	/*
> +	 * PARange[3:0] allows 0~5, other values are reserved,
> +	 * convert to physical address bits with a simple formula.
> +	 */
> +	u32 parange = read_system_reg(SYS_ID_AA64MMFR0_EL1) & 0xf;
> +
> +	return parange < 3 ? 32 + parange * 4 : 42 + (parange - 3) * (parange - 2);

This breaks for 52bit PA support added in ARMv8.2 [1]. It may be a good idea
to use an array here.

> +}
> +
>   #endif /* __ASSEMBLY__ */
>
>   #endif
> diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
> index 212ae63..f6b49db 100644
> --- a/arch/arm64/kernel/cpuinfo.c
> +++ b/arch/arm64/kernel/cpuinfo.c
> @@ -146,7 +146,9 @@ static int c_show(struct seq_file *m, void *v)
>   		seq_printf(m, "CPU architecture: 8\n");
>   		seq_printf(m, "CPU variant\t: 0x%x\n", MIDR_VARIANT(midr));
>   		seq_printf(m, "CPU part\t: 0x%03x\n", MIDR_PARTNUM(midr));
> -		seq_printf(m, "CPU revision\t: %d\n\n", MIDR_REVISION(midr));
> +		seq_printf(m, "CPU revision\t: %d\n", MIDR_REVISION(midr));
> +		seq_printf(m, "address sizes\t: %d bits physical, %d bits virtual\n\n",
> +			   id_aa64mmfr0_parange_bits(), VA_BITS);

VA_BITS is an attribute of your running kernel and doesn't have anything to do with what
the CPU can support. (e.g, it is 48bit on 8.0, 8.1, but could go upto 52 in 8.2)

[1] https://community.arm.com/groups/processors/blog/2016/01/05/armv8-a-architecture-evolution

Also, I am not sure if the change above would break userspace parsing the info. Ideally
it shouldn't , but...

Thanks
Suzuki