Re: [PATCH kernel 6/9] x86/dma-direct: Stop changing encrypted page state for TDISP devices

[Jason Gunthorpe]

On Mon, Mar 02, 2026 at 04:26:58PM +1100, Alexey Kardashevskiy wrote:

> > > Without secure vIOMMU, no Cbit in the S2 table (==host) for any
> > > VM. SDTE (==IOMMU) decides on shared/private for the device,
> > > i.e. (device_cc_accepted()?private:shared).
> > 
> > Is this "Cbit" part of the CPU S2 page table address space or is it
> > actually some PTE bit that says it is "encrypted" ?
> > 
> > It is confusing when you say it would start working with a vIOMMU.
> 
> When I mention vIOMMU, I mean the S1 table which is guest owned and
> which has Cbit in PTEs.

Yes, I understand this.

It seems from your email that the CPU S2 has the Cbit as part of the
address and the S1 feeds it through to the S2, so it is genuinely has
two addres spaces?

While the IOMMU S1 does not and instead needs a PTE bit which is
emphatically not an address bit because it does not feed through the
S2?

> > If 1<<51 is a valid IOPTE, and it is an actually address, then it
> > should be mapped into the IOMMU S2, shouldn't it? If it is in the
> > IOMMU S2 then shouldn't it work as a dma_addr_t?
> 
> It should (and checked with the HW folks), I just have not tried it  as, like, whyyy.

Well, I think things work more sensibly if you don't have to mangle
the address..

> > But in this case I would expect the vIOMMU to also use the same GPA
> > space starting from 0 and also remove the C bit, as the S2 shouldn't
> > have mappings starting at 1<<51.
> 
> How would then IOMMU know if DMA targets private or shared memory?
> The Cbit does not participate in the S2 translation as an address
> bit but IOMMU still knows what it is.

Same way it knows if there is no S1? Why does the S1 change anything?

> > > There is vTOM in SDTE which is "every phys_addr_t above vTOM is no
> > > Cbit, below - with Cbit" (and there is the same thing for the CPU
> > > side in SEV) but this not it, right?
> > 
> > That seems like the IOMMU HW is specially handling the address bits in
> > some way?
> 
> Yeah there is this capability. Except everything below vTOM is
> private and every above is shared so SME mask for it would be
> reverse than the CPU SME mask :) Not using this thing though (not
> sure why we have it). Thanks,

Weird!!

Jason