Re: [RFC PATCH v2 2/5] dma-mapping: Use the correct phys_to_dma() for DMA_RESTRICTED_POOL

Mostafa Saleh
2026-03-30T20:47:47Z

On Mon, Mar 30, 2026 at 12:09:03PM -0300, Jason Gunthorpe wrote:
> On Mon, Mar 30, 2026 at 02:50:40PM +0000, Mostafa Saleh wrote:
> > As restricted dma pools are always decrypted, in swiotlb.c it uses
> > phys_to_dma_unencrypted() for address conversion.
> >
> > However, in DMA-direct, calls to phys_to_dma_direct() with
> > force_dma_unencrypted() returning false, will fallback to
> > phys_to_dma() which is inconsistent for memory allocated from
> > restricted dma pools.
> >
> > Fixes: f4111e39a52a ("swiotlb: Add restricted DMA alloc/free support")
> > Signed-off-by: Mostafa Saleh <smostafa@google.com>
> > ---
> > kernel/dma/direct.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> > index 27d804f0473f..1a402bb956d9 100644
> > --- a/kernel/dma/direct.c
> > +++ b/kernel/dma/direct.c
> > @@ -26,7 +26,7 @@ u64 zone_dma_limit __ro_after_init = DMA_BIT_MASK(24);
> > static inline dma_addr_t phys_to_dma_direct(struct device *dev,
> > phys_addr_t phys)
> > {
> > - if (force_dma_unencrypted(dev))
> > + if (force_dma_unencrypted(dev) || is_swiotlb_for_alloc(dev))
> > return phys_to_dma_unencrypted(dev, phys);
>
> Same remark, I think the force_dma_unencrypted() was a hack to make up
> for a flag here. In these lower layers we need to annotate if phys is
> encrypted/decrypted and take the proper action universially.
>
> The force_dma_unencrypted() should only be done way up the call chain
> where we decide to get a phys that is decrypted. Once we have a
> decrypted phys it should be carried with an annotation throughout all
> the other places.

Can you please clarify what you mean by annotation in this context?
As I believe any tracking in the vmemmap is a big NO.

As replied to the first patch I can attempt to implement this approach
(by passing a flag around) and see how intrusive it would be.

Thanks,
Mostafa

>
> Then this is more like:
>
> if (flags & FLAG_DECRYPTED)
> return phys_to_dma_unencrypted(dev, phys);
>

> Jason