Subject: Re: [PATCH v9 07/14] swiotlb: Bounce data from/to restricted DMA pool if available

From: Christoph Hellwig <[email-masked]>
Date: Mon, 14 Jun 2021 08:25:30 +0200

On Fri, Jun 11, 2021 at 11:26:52PM +0800, Claire Chang wrote:
> Regardless of swiotlb setting, the restricted DMA pool is preferred if
> available.
> 
> The restricted DMA pools provide a basic level of protection against the
> DMA overwriting buffer contents at unexpected times. However, to protect
> against general data leakage and system memory corruption, the system
> needs to provide a way to lock down the memory access, e.g., MPU.
> 
> Note that is_dev_swiotlb_force doesn't check if
> swiotlb_force == SWIOTLB_FORCE. Otherwise the memory allocation behavior
> with default swiotlb will be changed by the following patche
> ("dma-direct: Allocate memory from restricted DMA pool if available").
> 
> Signed-off-by: Claire Chang <[email-masked]>
> ---
>  include/linux/swiotlb.h | 10 +++++++++-
>  kernel/dma/direct.c     |  3 ++-
>  kernel/dma/direct.h     |  3 ++-
>  kernel/dma/swiotlb.c    |  1 +
>  4 files changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index 06cf17a80f5c..8200c100fe10 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -85,6 +85,7 @@ extern enum swiotlb_force swiotlb_force;
>   *		unmap calls.
>   * @debugfs:	The dentry to debugfs.
>   * @late_alloc:	%true if allocated using the page allocator
> + * @force_swiotlb: %true if swiotlb is forced
>   */
>  struct io_tlb_mem {
>  	phys_addr_t start;
> @@ -95,6 +96,7 @@ struct io_tlb_mem {
>  	spinlock_t lock;
>  	struct dentry *debugfs;
>  	bool late_alloc;
> +	bool force_swiotlb;
>  	struct io_tlb_slot {
>  		phys_addr_t orig_addr;
>  		size_t alloc_size;
> @@ -115,6 +117,11 @@ static inline void swiotlb_set_io_tlb_default_mem(struct device *dev)
>  	dev->dma_io_tlb_mem = io_tlb_default_mem;
>  }
>  
> +static inline bool is_dev_swiotlb_force(struct device *dev)
> +{
> +	return dev->dma_io_tlb_mem->force_swiotlb;
> +}
> +
>  void __init swiotlb_exit(void);
>  unsigned int swiotlb_max_segment(void);
>  size_t swiotlb_max_mapping_size(struct device *dev);
> @@ -126,8 +133,9 @@ static inline bool is_swiotlb_buffer(struct device *dev, phys_addr_t paddr)
>  {
>  	return false;
>  }
> -static inline void swiotlb_set_io_tlb_default_mem(struct device *dev)
> +static inline bool is_dev_swiotlb_force(struct device *dev)
>  {
> +	return false;
>  }
>  static inline void swiotlb_exit(void)
>  {
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 7a88c34d0867..078f7087e466 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -496,7 +496,8 @@ size_t dma_direct_max_mapping_size(struct device *dev)
>  {
>  	/* If SWIOTLB is active, use its maximum mapping size */
>  	if (is_swiotlb_active(dev) &&
> -	    (dma_addressing_limited(dev) || swiotlb_force == SWIOTLB_FORCE))
> +	    (dma_addressing_limited(dev) || swiotlb_force == SWIOTLB_FORCE ||
> +	     is_dev_swiotlb_force(dev)))

I think we can remove the extra swiotlb_force check here if the
swiotlb_force setting is propagated into io_tlb_default_mem->force
when that is initialized. This avoids an extra check in the fast path.

> -	if (unlikely(swiotlb_force == SWIOTLB_FORCE))
> +	if (unlikely(swiotlb_force == SWIOTLB_FORCE) ||
> +	    is_dev_swiotlb_force(dev))

Same here.