Re: [PATCH v12 06/12] swiotlb: Use is_swiotlb_force_bounce for swiotlb data bouncing

[Stefano Stabellini]

On Wed, 16 Jun 2021, Claire Chang wrote:
> Propagate the swiotlb_force into io_tlb_default_mem->force_bounce and
> use it to determine whether to bounce the data or not. This will be
> useful later to allow for different pools.
> 
> Signed-off-by: Claire Chang <tientzu@xxxxxxxxxxxx>
> ---
>  include/linux/swiotlb.h | 11 +++++++++++
>  kernel/dma/direct.c     |  2 +-
>  kernel/dma/direct.h     |  2 +-
>  kernel/dma/swiotlb.c    |  4 ++++
>  4 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index dd1c30a83058..8d8855c77d9a 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -84,6 +84,7 @@ extern enum swiotlb_force swiotlb_force;
>   *		unmap calls.
>   * @debugfs:	The dentry to debugfs.
>   * @late_alloc:	%true if allocated using the page allocator
> + * @force_bounce: %true if swiotlb bouncing is forced
>   */
>  struct io_tlb_mem {
>  	phys_addr_t start;
> @@ -94,6 +95,7 @@ struct io_tlb_mem {
>  	spinlock_t lock;
>  	struct dentry *debugfs;
>  	bool late_alloc;
> +	bool force_bounce;
>  	struct io_tlb_slot {
>  		phys_addr_t orig_addr;
>  		size_t alloc_size;
> @@ -109,6 +111,11 @@ static inline bool is_swiotlb_buffer(struct device *dev, phys_addr_t paddr)
>  	return mem && paddr >= mem->start && paddr < mem->end;
>  }
>  
> +static inline bool is_swiotlb_force_bounce(struct device *dev)
> +{
> +	return dev->dma_io_tlb_mem->force_bounce;
> +}
>  void __init swiotlb_exit(void);
>  unsigned int swiotlb_max_segment(void);
>  size_t swiotlb_max_mapping_size(struct device *dev);
> @@ -120,6 +127,10 @@ static inline bool is_swiotlb_buffer(struct device *dev, phys_addr_t paddr)
>  {
>  	return false;
>  }
> +static inline bool is_swiotlb_force_bounce(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..a92465b4eb12 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -496,7 +496,7 @@ 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) || is_swiotlb_force_bounce(dev)))
>  		return swiotlb_max_mapping_size(dev);
>  	return SIZE_MAX;
>  }
> diff --git a/kernel/dma/direct.h b/kernel/dma/direct.h
> index 13e9e7158d94..4632b0f4f72e 100644
> --- a/kernel/dma/direct.h
> +++ b/kernel/dma/direct.h
> @@ -87,7 +87,7 @@ static inline dma_addr_t dma_direct_map_page(struct device *dev,
>  	phys_addr_t phys = page_to_phys(page) + offset;
>  	dma_addr_t dma_addr = phys_to_dma(dev, phys);
>  
> -	if (unlikely(swiotlb_force == SWIOTLB_FORCE))
> +	if (is_swiotlb_force_bounce(dev))
>  		return swiotlb_map(dev, phys, size, dir, attrs);
>
>  	if (unlikely(!dma_capable(dev, dma_addr, size, true))) {

Should we also make the same change in
drivers/xen/swiotlb-xen.c:xen_swiotlb_map_page ?

If I make that change, I can see that everything is working as
expected for a restricted-dma device with Linux running as dom0 on Xen.
However, is_swiotlb_force_bounce returns non-zero even for normal
non-restricted-dma devices. That shouldn't happen, right?

It looks like struct io_tlb_slot is not zeroed on allocation.
Adding memset(mem, 0x0, struct_size) in swiotlb_late_init_with_tbl
solves the issue.

With those two changes, the series passes my tests and you can add my
tested-by.