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

[Robin Murphy]

On 2026-02-25 4:30 pm, dan.j.williams@xxxxxxxxx wrote:
> 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 that we'd need a whole other private SWIOTLB plus the logic to 
decide which one to use in the first place. And if you really wanted an 
option to forcibly expose all DMA through shared memory regardless of 
TDISP and friends, that would logically want to be a higher-level CoCo 
option rather than belonging to SWIOTLB itself ;)

Thanks,
Robin.

> 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);