Subject: Re: [PATCH v14 07/44] arm64: RMI: Configure the RMM with the host's page size

From: Marc Zyngier <[email-masked]>
Date: Thu, 21 May 2026 14:30:37 +0100

On Wed, 13 May 2026 14:17:15 +0100,
Steven Price <[email-masked]> wrote:
> 
> RMM v2.0 brings the ability to set the RMM's granule size. Check the
> feature registers and configure the RMM so that it matches the host's
> page size. This means that operations can be done with a granulatity
> equal to PAGE_SIZE.
> 
> Signed-off-by: Steven Price <[email-masked]>
> ---
> Changes since v13:
>  * Moved out of KVM.
> ---
>  arch/arm64/kernel/rmi.c | 42 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
> 
> diff --git a/arch/arm64/kernel/rmi.c b/arch/arm64/kernel/rmi.c
> index 99c1ccc35c11..a14ead5dedda 100644
> --- a/arch/arm64/kernel/rmi.c
> +++ b/arch/arm64/kernel/rmi.c
> @@ -49,6 +49,45 @@ static int rmi_check_version(void)
>  	return 0;
>  }
>  
> +static int rmi_configure(void)
> +{
> +	struct rmm_config *config __free(free_page) = NULL;
> +	unsigned long ret;
> +
> +	config = (struct rmm_config *)get_zeroed_page(GFP_KERNEL);
> +	if (!config)
> +		return -ENOMEM;

This is the sort of buggy construct that is highlighted in
include/linux/cleanup.h: initialising the object for cleanup with
NULL, and only later assigning the expected value.

It may not matter here, but it will catch you (or more probably me) in
the future.

> +
> +	switch (PAGE_SIZE) {
> +	case SZ_4K:
> +		config->rmi_granule_size = RMI_GRANULE_SIZE_4KB;
> +		break;
> +	case SZ_16K:
> +		config->rmi_granule_size = RMI_GRANULE_SIZE_16KB;
> +		break;
> +	case SZ_64K:
> +		config->rmi_granule_size = RMI_GRANULE_SIZE_64KB;
> +		break;
> +	default:
> +		pr_err("Unsupported PAGE_SIZE for RMM\n");

Do you really anticipate PAGE_SIZE being any other value? This is 100%
dead code. If you want to be extra cautious, have a BUILD_BUg_ON().

> +		return -EINVAL;
> +	}
> +
> +	ret = rmi_rmm_config_set(virt_to_phys(config));
> +	if (ret) {
> +		pr_err("RMM config set failed\n");
> +		return -EINVAL;
> +	}

What is the live cycle of the page when the call succeeds? Is it
switched back to the NS PAS and allowed to be freed?

> +
> +	ret = rmi_rmm_activate();
> +	if (ret) {
> +		pr_err("RMM activate failed\n");
> +		return -ENXIO;
> +	}
> +
> +	return 0;
> +}
> +
>  static int __init arm64_init_rmi(void)
>  {
>  	/* Continue without realm support if we can't agree on a version */
> @@ -60,6 +99,9 @@ static int __init arm64_init_rmi(void)
>  	if (WARN_ON(rmi_features(1, &rmm_feat_reg1)))
>  		return 0;
>  
> +	if (rmi_configure())
> +		return 0;
> +
>  	return 0;
>  }
>  subsys_initcall(arm64_init_rmi);

Thanks,

	M.

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