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

[Kohei Enju]

Good day Gavin,

On 09/08 20:59, Gavin Shan wrote:
> 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@xxxxxxx>
> > > > 
> > > > 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@xxxxxxx>
> > > > Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> > > > ---
> > > >    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,

Indeed. I understand your point.

> 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 for the detailed explanation.

Thanks,
Kohei

> 
> 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
> > > 
> > 
>