Re: [PATCH v6 3/7] arm64: cpufeature: discover CPU support for MPAM

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Oct 30, 2024 at 04:03:13PM +0000, Joey Gouly wrote:
> From: James Morse <james.morse@arm.com>
> 
> 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)
> 
> Signed-off-by: James Morse <james.morse@arm.com>
> Signed-off-by: Joey Gouly <joey.gouly@arm.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>

Acked-by: Catalin Marinas <catalin.marinas@arm.com>