[PATCH v3 2/6] arm64: cpufeature: discover CPU support for MPAM

[Will Deacon]

Hi James,

On Thu, Mar 21, 2024 at 04:57:24PM +0000, James Morse wrote:
> ARMv8.4 adds support for 'Memory Partitioning And Monitoring' (MPAM)
> which describes an interface to cache and bandwidth controls wherever
> they appear in the system.
> 
> Add support to detect MPAM. Like SVE, MPAM has an extra id register that
> describes some more properties, including the virtualisation support,
> which is optional. Detect this separately so we can detect
> mismatched/insane systems, but still use MPAM on the host even if the
> virtualisation support is missing.
> 
> MPAM needs enabling at the highest implemented exception level, otherwise
> the register accesses trap. The 'enabled' flag is accessible to lower
> exception levels, but its in a register that traps when MPAM isn't enabled.
> The cpufeature 'matches' hook is extended to test this on one of the
> CPUs, so that firmware can emulate MPAM as disabled if it is reserved
> for use by secure world.
> 
> Secondary CPUs that appear late could trip cpufeature's 'lower safe'
> behaviour after the MPAM properties have been advertised to user-space.
> Add a verify call to ensure late secondaries match the existing CPUs.
> 
> (If you have a boot failure that bisects here its likely your CPUs
> advertise MPAM in the id registers, but firmware failed to either enable
> or MPAM, or emulate the trap as if it were disabled)

I'm probably reading this wrong, but I'm a bit confused about the mixture
of open-coded system register definitions and updates to the 'sysreg'
file. For example:

> +/* CPU Registers */
> +#define MPAM_SYSREG_EN			BIT_ULL(63)
> +#define MPAM_SYSREG_TRAP_IDR		BIT_ULL(58)
> +#define MPAM_SYSREG_TRAP_MPAM0_EL1	BIT_ULL(49)
> +#define MPAM_SYSREG_TRAP_MPAM1_EL1	BIT_ULL(48)
> +#define MPAM_SYSREG_PMG_D		GENMASK(47, 40)
> +#define MPAM_SYSREG_PMG_I		GENMASK(39, 32)
> +#define MPAM_SYSREG_PARTID_D		GENMASK(31, 16)
> +#define MPAM_SYSREG_PARTID_I		GENMASK(15, 0)

MPAM_SYSREG_EN is then used in conjuntion with SYS_MPAM1_EL1:

> +static bool __maybe_unused
> +test_has_mpam(const struct arm64_cpu_capabilities *entry, int scope)
> +{
> +	if (!has_cpuid_feature(entry, scope))
> +		return false;
> +
> +	/* Check firmware actually enabled MPAM on this cpu. */
> +	return (read_sysreg_s(SYS_MPAM1_EL1) & MPAM_SYSREG_EN);
> +}

But that register has a 'sysreg' entry:

> +Sysreg	MPAM1_EL1	3	0	10	5	0
> +Res0	63:48
> +Field	47:40	PMG_D
> +Field	39:32	PMG_I
> +Field	31:16	PARTID_D
> +Field	15:0	PARTID_I
> +EndSysreg

where bit 63 is RES0.

Will