Re: [PATCH v2] KVM: arm64: Trap guest MPAM accesses whenever MPAM is implemented

[Marc Zyngier <maz@kernel.org>]

On Tue, 08 Sep 2026 15:56:51 +0100,
Fuad Tabba <fuad.tabba@linux.dev> wrote:
> 
> finalise_el2_state() clears the EL2 MPAM traps whenever the ID
> registers advertise MPAM, while KVM sets them only under ARM64_MPAM,
> which also requires MPAMEN. Without EL3 nothing sets that enable, so a
> guest reaches the MPAM registers while ID_AA64PFR0_EL1.MPAM reads 0
> for it.
> 
> Gate the traps on the ID registers alone: MPAMEN isn't a term in any
> MPAM accessor, so they take effect without it. The cap tests the
> sanitised ID fields, which never exceed a CPU's own, so it's set only
> where finalise_el2_state already wrote MPAM2_EL2. MPAM3_EL3.TRAPLOWER
> is clear there, and arm64.nompam turns the cap off too.
> 
> Fixes: 23b33d1e168c ("arm64: head.S: Initialise MPAM EL2 registers and disable traps")
> Reviewed-by: Yuan Yao <yaoyuan@linux.alibaba.com>
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
> ---
> 
> Notes:
>     Changes since v1:
>      - Commit message only. The code is unchanged, so Yuan Yao's
>        Reviewed-by is carried.
>      - Fixes: names the commit that cleared the traps, rather than the
>        one that added KVM's trapping.
>      - Says why the sanitised ID fields are the safe gate: the cap is set
>        only where finalise_el2_state already wrote MPAM2_EL2.
>     
>     Not covered, and answered on the v1 thread: a machine whose CPUs
>     differ on MPAM, raised by Ben and by sashiko.
>     
>     v1: https://lore.kernel.org/all/20260903160819.831518-1-fuad.tabba@linux.dev/
> 
>  arch/arm64/include/asm/cpufeature.h     |  5 +++++
>  arch/arm64/kernel/cpufeature.c          | 13 +++++++++++++
>  arch/arm64/kvm/hyp/include/hyp/switch.h |  4 ++--
>  arch/arm64/tools/cpucaps                |  1 +
>  4 files changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 7404a6e83a930..8863ae99596bc 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -873,6 +873,11 @@ static __always_inline bool system_supports_mpam_hcr(void)
>  	return alternative_has_cap_unlikely(ARM64_MPAM_HCR);
>  }
>  
> +static __always_inline bool system_supports_mpam_sysregs(void)
> +{
> +	return alternative_has_cap_unlikely(ARM64_MPAM_SYSREGS);
> +}
> +

What is the rationale for choosing alternative_has_cap_unlikely()
instead of cpus_have_final_cap(), which is more wildly used?

	M.

-- 
Without deviation from the norm, progress is not possible.