Re: [PATCH] arm64: Add kernel parameter to disable trap EL0 accesses to IMPDEF regs

[Yicong Yang <yangyccccc@gmail.com>]

On 2025/10/21 19:54, Liao Chang wrote:
> Add kernel parameter to allow system-wide EL0 access to IMPDEF system
> regregisters and instructions without trapping to EL1/EL2. Since trap
> overhead will compromises benefits, and it's even worse in
> virtualization on CPU where certain IMPDEF registers and instructions
> are designed for EL0 performance use.
>
> More early discussion could be found from link below.
>
> Link: https://lore.kernel.org/all/24afb8de-622a-4865-bd8e-8e89ccfff8f4@huawei.com/
> Tested-by: Xinyu Zheng <zhengxinyu6@huawei.com>
> Signed-off-by: Liao Chang <liaochang1@huawei.com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  9 +++++++++
>  arch/arm64/kernel/cpufeature.c                  | 14 +++++++++++++-
>  2 files changed, 22 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 5a7a83c411e9..11ffa9f7b972 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -326,6 +326,15 @@
>  			See Documentation/arch/arm64/asymmetric-32bit.rst for more
>  			information.
>  
> +	no_trap_el0_impdef [Arm64,EARLY]
> +			Allow system-wide EL0 access to IMPDEF system registers
> +			and instructions without trapping to EL1/EL2.
> +			Since trap overhead compromises benefits, and it's even
> +			worse in virtualization on CPU where certain IMPDEF
> +			registers and instructions are designed for EL0
> +			performance use. This assumes the kernel adds the
> +			support for Armv8.8 extension FEAT_TIDCP1.
> +
>  	amd_iommu=	[HW,X86-64]
>  			Pass parameters to the AMD IOMMU driver in the system.
>  			Possible values are:
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index ef269a5a37e1..d12e35d799ee 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -2459,9 +2459,21 @@ static bool is_kvm_protected_mode(const struct arm64_cpu_capabilities *entry, in
>  }
>  #endif /* CONFIG_KVM */
>  
> +static bool no_trap_el0_impdef;
> +
> +static int __init parse_no_trap_el0_impdef(char *p)
> +{
> +	no_trap_el0_impdef = true;
> +	return 0;
> +}
> +early_param("no_trap_el0_impdef", parse_no_trap_el0_impdef);
> +
>  static void cpu_trap_el0_impdef(const struct arm64_cpu_capabilities *__unused)
>  {
> -	sysreg_clear_set(sctlr_el1, 0, SCTLR_EL1_TIDCP);
> +	if (no_trap_el0_impdef)
> +		sysreg_clear_set(sctlr_el1, SCTLR_EL1_TIDCP, 0);
> +	else
> +		sysreg_clear_set(sctlr_el1, 0, SCTLR_EL1_TIDCP);
An id override for TIDCP1 seems simpler rather than an early param, and consistent to how we disable other 
certain features through boot option like arm64.nosve or arm64.nompam. I see Catalin also suggested in that
way in [1],any reason we cannot do it in that way?

[1] https://lore.kernel.org/all/aHqamaqueuk18NyS@arm.com/

Thanks.
>  }
>  
>  static void cpu_enable_dit(const struct arm64_cpu_capabilities *__unused)