linux-kernel - Re: [PATCH v3 07/47] arm64: mpam: Re-initialise MPAM regs when CPU comes online

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Message-ID: <aWkucXgE6g2lwvkj@arm.com>
Date: Thu, 15 Jan 2026 18:14:09 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Ben Horgan <ben.horgan@....com>
Cc: amitsinght@...vell.com, baisheng.gao@...soc.com,
	baolin.wang@...ux.alibaba.com, carl@...amperecomputing.com,
	dave.martin@....com, david@...nel.org, dfustini@...libre.com,
	fenghuay@...dia.com, gshan@...hat.com, james.morse@....com,
	jonathan.cameron@...wei.com, kobak@...dia.com, lcherian@...vell.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	peternewman@...gle.com, punit.agrawal@....qualcomm.com,
	quic_jiles@...cinc.com, reinette.chatre@...el.com,
	rohit.mathew@....com, scott@...amperecomputing.com,
	sdonthineni@...dia.com, tan.shaopeng@...itsu.com,
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	maz@...nel.org, oupton@...nel.org, joey.gouly@....com,
	suzuki.poulose@....com, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v3 07/47] arm64: mpam: Re-initialise MPAM regs when CPU
 comes online

On Mon, Jan 12, 2026 at 04:58:34PM +0000, Ben Horgan wrote:
> From: James Morse <james.morse@....com>
> 
> Now that the MPAM system registers are expected to have values that change,
> reprogram them based on the previous value when a CPU is brought online.
> 
> Previously MPAM's 'default PARTID' of 0 was always used for MPAM in
> kernel-space as this is the PARTID that hardware guarantees to
> reset. Because there are a limited number of PARTID, this value is exposed
> to user-space, meaning resctrl changes to the resctrl default group would
> also affect kernel threads.  Instead, use the task's PARTID value for
> kernel work on behalf of user-space too. The default of 0 is kept for both
> user-space and kernel-space when MPAM is not enabled.
> 
> Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.com>
> Signed-off-by: James Morse <james.morse@....com>
> Signed-off-by: Ben Horgan <ben.horgan@....com>
> ---
> Changes since rfc:
> CONFIG_MPAM -> CONFIG_ARM64_MPAM
> Check mpam_enabled
> Comment about relying on ERET for synchronisation
> Update commit message
> ---
>  arch/arm64/kernel/cpufeature.c | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index c840a93b9ef9..0cdfb3728f43 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -86,6 +86,7 @@
>  #include <asm/kvm_host.h>
>  #include <asm/mmu.h>
>  #include <asm/mmu_context.h>
> +#include <asm/mpam.h>
>  #include <asm/mte.h>
>  #include <asm/hypervisor.h>
>  #include <asm/processor.h>
> @@ -2483,13 +2484,17 @@ test_has_mpam(const struct arm64_cpu_capabilities *entry, int scope)
>  static void
>  cpu_enable_mpam(const struct arm64_cpu_capabilities *entry)
>  {
> -	/*
> -	 * Access by the kernel (at EL1) should use the reserved PARTID
> -	 * which is configured unrestricted. This avoids priority-inversion
> -	 * where latency sensitive tasks have to wait for a task that has
> -	 * been throttled to release the lock.
> -	 */
> -	write_sysreg_s(0, SYS_MPAM1_EL1);

Is this comment about priority inversion no longer valid? I see thread
switching sets the same value for both MPAM0 and MPAM1 registers but I
couldn't find an explanation why this is now better when it wasn't
before.

MPAM1 will also be inherited by IRQ handlers AFAICT.

> +	int cpu = smp_processor_id();
> +	u64 regval = 0;
> +
> +	if (IS_ENABLED(CONFIG_ARM64_MPAM) && static_branch_likely(&mpam_enabled))
> +		regval = READ_ONCE(per_cpu(arm64_mpam_current, cpu));
> +
> +	write_sysreg_s(regval, SYS_MPAM1_EL1);
> +	isb();
> +
> +	/* Synchronising the EL0 write is left until the ERET to EL0 */
> +	write_sysreg_s(regval, SYS_MPAM0_EL1);

I mentioned before, is it worth waiting until ERET?

Related to this, do LDTR/STTR use MPAM0 or MPAM1? I couldn't figure out
from the Arm ARM. If they use MPAM0, then we need the ISB early for the
uaccess routines, at least in the thread switching path (an earlier
patch).

-- 
Catalin