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

[Alexey Kardashevskiy]

On 4/3/26 23:43, Jason Gunthorpe wrote:
> On Wed, Mar 04, 2026 at 05:45:31PM +1100, Alexey Kardashevskiy wrote:
>
> > > I suspect AMD needs to use their vTOM feature to allow shared memory
> > > to remain available to TDISP RUN with a high/low address split.
> >
> > I could probably do something about it bit I wonder what is the real
> > live use case which requires leaking SME mask, have a live example
> > which I could try recreating?
>
> We need shared memory allocated through a DMABUF heap:
>
> https://lore.kernel.org/all/20260223095136.225277-1-jiri@xxxxxxxxxxx/
>
> To work with all PCI devices in the system, TDISP or not.
>
> Without this the ability for a TDISP device to ingest (encrypted) data
> requires all kinds of memcpy..
>
> So the DMA API should see the DMA_ATTR_CC_DECRYPTED and setup the
> correct dma_dddr_t either by choosing the shared alias for the TDISP
> device's vTOM, or setting the C bit in a vIOMMU S1.

Something like that?

https://github.com/AMDESE/linux-kvm/commit/266a41a1ea746557eb63debce886ce2c98820667

With some little hacks I can make this tree do TDISP DMA to private or shared (swiotlb) memory by steering via this vTOM thing. Thanks,



> Jason

--
Alexey