Re: [PATCH kernel 4/9] dma/swiotlb: Stop forcing SWIOTLB for TDISP devices

[dan.j.williams]

Alexey Kardashevskiy wrote:
> SWIOTLB is enforced when encrypted guest memory is detected
> in pci_swiotlb_detect() which is required for legacy devices.
> 
> Skip SWIOTLB for TDISP devices.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxx>
> ---
>  include/linux/swiotlb.h | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index 3dae0f592063..119c25d639a7 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -173,6 +173,15 @@ static inline bool is_swiotlb_force_bounce(struct device *dev)
>  {
>  	struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
>  
> +	/*
> +	 * CC_ATTR_GUEST_MEM_ENCRYPT enforces SWIOTLB_FORCE in
> +	 * swiotlb_init_remap() to allow legacy devices access arbitrary
> +	 * VM encrypted memory.
> +	 * Skip it for TDISP devices capable of DMA-ing the encrypted memory.
> +	 */
> +	if (device_cc_accepted(dev))
> +		return false;

I worry this further muddies the meaning of the swiotlb force option.
What if you want to force swiotlb operation on accepted devices?

For example:

@@ -173,7 +176,13 @@ static inline bool is_swiotlb_force_bounce(struct device *dev)
 {
        struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
 
-       return mem && mem->force_bounce;
+       if (!mem)
+               return false;
+       if (mem->force_bounce)
+               return true;
+       if (mem->bounce_unaccepted && !device_cc_accepted(dev))
+               return true;
+       return false;
 }
 
 void swiotlb_init(bool addressing_limited, unsigned int flags);