Re: [PATCH] iommu: Add support to filter non-strict/lazy mode based on device names

[Robin Murphy <robin.murphy@arm.com>]

On 2020-08-25 16:42, Sai Prakash Ranjan wrote:
> Currently the non-strict or lazy mode of TLB invalidation can only be set
> for all or no domains. This works well for development platforms where
> setting to non-strict/lazy mode is fine for performance reasons but on
> production devices, we need a more fine grained control to allow only
> certain peripherals to support this mode where we can be sure that it is
> safe. So add support to filter non-strict/lazy mode based on the device
> names that are passed via cmdline parameter "iommu.nonstrict_device".

There seems to be considerable overlap here with both the existing 
patches for per-device default domain control [1], and the broader 
ongoing development on how to define, evaluate and handle "trusted" vs. 
"untrusted" devices (e.g. [2],[3]). I'd rather see work done to make 
sure those integrate properly together and work well for everyone's 
purposes, than add more disjoint mechanisms that only address small 
pieces of the overall issue.

Robin.

[1] 
https://lore.kernel.org/linux-iommu/20200824051726.7xaJRTTszJuzdFWGJ8YNsshCtfNR0BNeMrlILAyqt_0@z/
[2] 
https://lore.kernel.org/linux-iommu/20200630044943.3425049-1-rajatja@google.com/
[3] 
https://lore.kernel.org/linux-iommu/20200626002710.110200-2-rajatja@google.com/

> Example: iommu.nonstrict_device="7c4000.sdhci,a600000.dwc3,6048000.etr"
> 
> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
> ---
>   drivers/iommu/iommu.c | 37 +++++++++++++++++++++++++++++++++----
>   1 file changed, 33 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 609bd25bf154..fd10a073f557 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -32,6 +32,9 @@ static unsigned int iommu_def_domain_type __read_mostly;
>   static bool iommu_dma_strict __read_mostly = true;
>   static u32 iommu_cmd_line __read_mostly;
>   
> +#define DEVICE_NAME_LEN		1024
> +static char nonstrict_device[DEVICE_NAME_LEN] __read_mostly;
> +
>   struct iommu_group {
>   	struct kobject kobj;
>   	struct kobject *devices_kobj;
> @@ -327,6 +330,32 @@ static int __init iommu_dma_setup(char *str)
>   }
>   early_param("iommu.strict", iommu_dma_setup);
>   
> +static int __init iommu_nonstrict_filter_setup(char *str)
> +{
> +	strlcpy(nonstrict_device, str, DEVICE_NAME_LEN);
> +	return 1;
> +}
> +__setup("iommu.nonstrict_device=", iommu_nonstrict_filter_setup);
> +
> +static bool iommu_nonstrict_device(struct device *dev)
> +{
> +	char *filter, *device;
> +
> +	if (!dev)
> +		return false;
> +
> +	filter = kstrdup(nonstrict_device, GFP_KERNEL);
> +	if (!filter)
> +		return false;
> +
> +	while ((device = strsep(&filter, ","))) {
> +		if (!strcmp(device, dev_name(dev)))
> +			return true;
> +	}
> +
> +	return false;
> +}
> +
>   static ssize_t iommu_group_attr_show(struct kobject *kobj,
>   				     struct attribute *__attr, char *buf)
>   {
> @@ -1470,7 +1499,7 @@ static int iommu_get_def_domain_type(struct device *dev)
>   
>   static int iommu_group_alloc_default_domain(struct bus_type *bus,
>   					    struct iommu_group *group,
> -					    unsigned int type)
> +					    unsigned int type, struct device *dev)
>   {
>   	struct iommu_domain *dom;
>   
> @@ -1489,7 +1518,7 @@ static int iommu_group_alloc_default_domain(struct bus_type *bus,
>   	if (!group->domain)
>   		group->domain = dom;
>   
> -	if (!iommu_dma_strict) {
> +	if (!iommu_dma_strict || iommu_nonstrict_device(dev)) {
>   		int attr = 1;
>   		iommu_domain_set_attr(dom,
>   				      DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE,
> @@ -1509,7 +1538,7 @@ static int iommu_alloc_default_domain(struct iommu_group *group,
>   
>   	type = iommu_get_def_domain_type(dev);
>   
> -	return iommu_group_alloc_default_domain(dev->bus, group, type);
> +	return iommu_group_alloc_default_domain(dev->bus, group, type, dev);
>   }
>   
>   /**
> @@ -1684,7 +1713,7 @@ static void probe_alloc_default_domain(struct bus_type *bus,
>   	if (!gtype.type)
>   		gtype.type = iommu_def_domain_type;
>   
> -	iommu_group_alloc_default_domain(bus, group, gtype.type);
> +	iommu_group_alloc_default_domain(bus, group, gtype.type, NULL);
>   
>   }
>   
> 
> base-commit: e46b3c0d011eab9933c183d5b47569db8e377281
> 
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