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

Mostafa Saleh
2026-04-15T20:27:24Z

On Mon, Apr 13, 2026 at 11:38:36AM +0530, Aneesh Kumar K.V wrote:
> Mostafa Saleh <smostafa@google.com> writes:
>
> > 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);
> >
>
> So what kind of device is this? Is it a trusted device that needs to use
> swiotlb in unencrypted form?(is that a valid use case?) Can we add additional comment
> explaining the type of device for which we are allocating the DMA
> buffer?

That’s used for devices that use restricted-dma pools which are
currently always decrypted, typically virtio devices that are emulated
by the untrusted host.

Thanks,
Mostafa

>
> > return phys_to_dma(dev, phys);
> > }
> > --
> > 2.53.0.1185.g05d4b7b318-goog
>
> -aneesh