Re: [PATCH] iommu/dma: Resurrect the "forcedac" option

[John Garry <john.garry@huawei.com>]

On 05/03/2021 16:32, Robin Murphy wrote:
> In converting intel-iommu over to the common IOMMU DMA ops, it quietly
> lost the functionality of its "forcedac" option. Since this is a handy
> thing both for testing and for performance optimisation on certain
> platforms, reimplement it under the common IOMMU parameter namespace.
> 
> For the sake of fixing the inadvertent breakage of the Intel-specific
> parameter, remove the dmar_forcedac remnants and hook it up as an alias
> while documenting the transition to the new common parameter.
> 

Do you think that having a kconfig option to control the default for 
this can help identify the broken platforms which rely on forcedac=0? 
But seems a bit trivial for that, though.

Or are we bothered (finding them)?

Thanks,
john

> Fixes: c588072bba6b ("iommu/vt-d: Convert intel iommu driver to the iommu ops")
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>   Documentation/admin-guide/kernel-parameters.txt | 15 ++++++++-------
>   drivers/iommu/dma-iommu.c                       | 13 ++++++++++++-
>   drivers/iommu/intel/iommu.c                     |  5 ++---
>   include/linux/dma-iommu.h                       |  2 ++
>   4 files changed, 24 insertions(+), 11 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 04545725f187..835f810f2f26 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1869,13 +1869,6 @@
>   			bypassed by not enabling DMAR with this option. In
>   			this case, gfx device will use physical address for
>   			DMA.
> -		forcedac [X86-64]
> -			With this option iommu will not optimize to look
> -			for io virtual address below 32-bit forcing dual
> -			address cycle on pci bus for cards supporting greater
> -			than 32-bit addressing. The default is to look
> -			for translation below 32-bit and if not available
> -			then look in the higher range.
>   		strict [Default Off]
>   			With this option on every unmap_single operation will
>   			result in a hardware IOTLB flush operation as opposed
> @@ -1964,6 +1957,14 @@
>   		nobypass	[PPC/POWERNV]
>   			Disable IOMMU bypass, using IOMMU for PCI devices.
>   
> +	iommu.forcedac=	[ARM64, X86] Control IOVA allocation for PCI devices.
> +			Format: { "0" | "1" }
> +			0 - Try to allocate a 32-bit DMA address first, before
> +			  falling back to the full range if needed.
> +			1 - Allocate directly from the full usable range,
> +			  forcing Dual Address Cycle for PCI cards supporting
> +			  greater than 32-bit addressing.
> +
>   	iommu.strict=	[ARM64] Configure TLB invalidation behaviour
>   			Format: { "0" | "1" }
>   			0 - Lazy mode.
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 9ab6ee22c110..260bf3de1992 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -52,6 +52,17 @@ struct iommu_dma_cookie {
>   };
>   
>   static DEFINE_STATIC_KEY_FALSE(iommu_deferred_attach_enabled);
> +bool iommu_dma_forcedac __read_mostly;
> +
> +static int __init iommu_dma_forcedac_setup(char *str)
> +{
> +	int ret = kstrtobool(str, &iommu_dma_forcedac);
> +
> +	if (!ret && iommu_dma_forcedac)
> +		pr_info("Forcing DAC for PCI devices\n");
> +	return ret;
> +}
> +early_param("iommu.forcedac", iommu_dma_forcedac_setup);
>   
>   void iommu_dma_free_cpu_cached_iovas(unsigned int cpu,
>   		struct iommu_domain *domain)
> @@ -438,7 +449,7 @@ static dma_addr_t iommu_dma_alloc_iova(struct iommu_domain *domain,
>   		dma_limit = min(dma_limit, (u64)domain->geometry.aperture_end);
>   
>   	/* Try to get PCI devices a SAC address */
> -	if (dma_limit > DMA_BIT_MASK(32) && dev_is_pci(dev))
> +	if (dma_limit > DMA_BIT_MASK(32) && !iommu_dma_forcedac && dev_is_pci(dev))
>   		iova = alloc_iova_fast(iovad, iova_len,
>   				       DMA_BIT_MASK(32) >> shift, false);
>   
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index ee0932307d64..1c32522220bc 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -360,7 +360,6 @@ int intel_iommu_enabled = 0;
>   EXPORT_SYMBOL_GPL(intel_iommu_enabled);
>   
>   static int dmar_map_gfx = 1;
> -static int dmar_forcedac;
>   static int intel_iommu_strict;
>   static int intel_iommu_superpage = 1;
>   static int iommu_identity_mapping;
> @@ -451,8 +450,8 @@ static int __init intel_iommu_setup(char *str)
>   			dmar_map_gfx = 0;
>   			pr_info("Disable GFX device mapping\n");
>   		} else if (!strncmp(str, "forcedac", 8)) {
> -			pr_info("Forcing DAC for PCI devices\n");
> -			dmar_forcedac = 1;
> +			pr_warn("intel_iommu=forcedac deprecated; use iommu.forcedac instead\n");
> +			iommu_dma_forcedac = true;
>   		} else if (!strncmp(str, "strict", 6)) {
>   			pr_info("Disable batched IOTLB flush\n");
>   			intel_iommu_strict = 1;
> diff --git a/include/linux/dma-iommu.h b/include/linux/dma-iommu.h
> index 706b68d1359b..13d1f4c14d7b 100644
> --- a/include/linux/dma-iommu.h
> +++ b/include/linux/dma-iommu.h
> @@ -40,6 +40,8 @@ void iommu_dma_get_resv_regions(struct device *dev, struct list_head *list);
>   void iommu_dma_free_cpu_cached_iovas(unsigned int cpu,
>   		struct iommu_domain *domain);
>   
> +extern bool iommu_dma_forcedac;
> +
>   #else /* CONFIG_IOMMU_DMA */
>   
>   struct iommu_domain;
>