Re: [PATCH v17 3/7] firmware: arm_rmm: Configure the RMM with the host's page size

[Gavin Shan <gshan@redhat.com>]

Hi Kohei,

On 9/8/26 6:00 PM, Kohei Enju wrote:
> Hi Gavin,
> 
> On 09/08 17:04, Gavin Shan wrote:
>> Hi Suzuki,
>>
>> On 9/7/26 7:59 PM, Suzuki K Poulose wrote:
>>> From: Steven Price <steven.price@arm.com>
>>>
>>> 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 granularity
>>> equal to PAGE_SIZE.
>>>
>>> Signed-off-by: Steven Price <steven.price@arm.com>
>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>>> ---
>>>    Changes since v15:
>>>     * Actually check the feature register for the host's page-size support.
>>>    Changes since v14:
>>>     * Move the implementation into drivers/firmware/arm_rmm.
>>>    Changes since v13:
>>>     * Moved out of KVM.
>>> ---
>>>    drivers/firmware/arm_rmm/rmi.c | 58 ++++++++++++++++++++++++++++++++++
>>>    include/linux/arm-rmi-cmds.h   | 17 ++++++++++
>>>    2 files changed, 75 insertions(+)
>>>
>>> diff --git a/drivers/firmware/arm_rmm/rmi.c b/drivers/firmware/arm_rmm/rmi.c
>>> index 008a783407b4e..76f91c145e1fd 100644
>>> --- a/drivers/firmware/arm_rmm/rmi.c
>>> +++ b/drivers/firmware/arm_rmm/rmi.c
>>> @@ -77,6 +77,60 @@ static int rmi_read_features(void)
>>>    	return 0;
>>>    }
>>> +static int rmi_configure(void)
>>> +{
>>> +	unsigned long granule_feature;
>>> +	unsigned long granule_size;
>>> +	int ret = 0;
>>> +	struct rmm_config *config;
>>> +
>>> +	switch (PAGE_SIZE) {
>>> +	case SZ_4K:
>>> +		granule_size = RMI_GRANULE_SIZE_4KB;
>>> +		granule_feature = RMI_FEATURE_REGISTER_1_RMI_GRAN_SZ_4KB;
>>> +		break;
>>> +	case SZ_16K:
>>> +		granule_size = RMI_GRANULE_SIZE_16KB;
>>> +		granule_feature = RMI_FEATURE_REGISTER_1_RMI_GRAN_SZ_16KB;
>>> +		break;
>>> +	case SZ_64K:
>>> +		granule_size = RMI_GRANULE_SIZE_64KB;
>>> +		granule_feature = RMI_FEATURE_REGISTER_1_RMI_GRAN_SZ_64KB;
>>> +		break;
>>> +	default:
>>> +		BUILD_BUG();
>>> +	}
>>> +
>>> +	if (!(rmi_feat_reg(1) & granule_feature)) {
>>> +		pr_err("RMM does not support %luKB granules\n",
>>> +		       PAGE_SIZE >> 10);
>>> +		return -ENXIO;
>>> +	}
>>> +
>>> +	config = (struct rmm_config *)get_zeroed_page(GFP_KERNEL);
>>> +	if (!config)
>>> +		return -ENOMEM;
>>
>> An error message is needed here.
>>
>> 	if (!config) {
>> 		pr_err("Unable to alloc RMM config memory\n");
>> 		return -ENOMEM;
>> 	}
> 
> I largely agree with your suggestions, and they look reasonable to me.
> 
> However, wouldn't this be redundant? Since __GFP_NOWARN is not set, the
> page allocator would normally emit an allocation failure warning with a
> stack trace anyway.
> 
> I don't have a strong preference, but as per "14) Allocating memory" in
> the coding style, I believe it would be considered unnecessary.
> 

You're correct that the error message is most likely redundant here and we
needn't it. However, it's notable the allocation failure warning and stack
trace isn't 100% raised when looking at mm/page_alloc.c::warn_alloc(). At least,
it depends on !__ratelimit(&nopage_rs) even it's less likely to happen. Similarly,
there are conditions to raise a warning and stack track in case of the injected
allocation error in lib/fault-inject.c::fail_dump().

Thanks,
Gavin

> Thanks,
> Kohei
> 
>>
>>> +
>>> +	config->rmi_granule_size = granule_size;
>>> +
>>> +	/*
>>> +	 * For now we set the tracking_region_size to 0 which is the only option
>>> +	 * for 4KB PAGE_SIZE (1GB for 4KB PAGE_SIZE, 32MB/512MB for 16KB/64KB).
>>> +	 * TODO: Support other tracking sizes via Kconfig option for other
>>> +	 * PAGE_SIZES
>>> +	 */
>>> +	config->tracking_region_size = 0;
>>> +
>>> +	ret = rmi_rmm_config_set(virt_to_phys(config));
>>> +	if (ret) {
>>> +		pr_err("RMM config set failed\n");
>>> +		ret = -EINVAL;
>>> +	}
>>
>> The error code from rmi_rmm_config_set() is indicative sometimes. Also, -ENXIO
>> would be more appropriate than -EINVAL?
>>
>> 	if (ret) {
>> 		pr_err("RMM config set failed (%d)\n", ret);
>> 		ret = -ENXIO;
>> 	}
>>
>>> +
>>> +	free_page((unsigned long)config);
>>> +	return ret;
>>> +}
>>> +
>>>    static int __init arm64_init_rmi(void)
>>>    {
>>>    	int ret;
>>> @@ -90,6 +144,10 @@ static int __init arm64_init_rmi(void)
>>>    	if (ret)
>>>    		return ret;
>>> +	ret = rmi_configure();
>>> +	if (ret)
>>> +		return ret;
>>> +
>>>    	return 0;
>>>    }
>>> diff --git a/include/linux/arm-rmi-cmds.h b/include/linux/arm-rmi-cmds.h
>>> index 2fb1f7f86d71e..9aa27697e2377 100644
>>> --- a/include/linux/arm-rmi-cmds.h
>>> +++ b/include/linux/arm-rmi-cmds.h
>>> @@ -12,6 +12,23 @@
>>>    unsigned long rmi_feat_reg(unsigned long id);
>>> +/**
>>> + * rmi_rmm_config_set() - Configure the RMM
>>> + * @cfg_ptr: PA of a struct rmm_config
>>> + *
>>> + * Sets configuration options on the RMM.
>>> + *
>>> + * Return: RMI return code
>>> + */
>>> +static inline int rmi_rmm_config_set(unsigned long cfg_ptr)
>>> +{
>>> +	struct arm_smccc_res res;
>>> +
>>> +	arm_smccc_1_1_invoke(SMC_RMI_RMM_CONFIG_SET, cfg_ptr, &res);
>>> +
>>> +	return res.a0;
>>> +}
>>> +
>>
>> rmi_rmm_config_set() is used for once by rmi.c::rmi_configure(), I would not expose
>> rmi_rmm_config_set() by combining the logic to rmi.c::rmi_configure().
>>
>>>    /**
>>>     * rmi_features() - Read feature register
>>>     * @index: Feature register index
>>
>> Thanks,
>> Gavin
>>
>