Re: [RFC PATCH 19/30] vfio/pci: Add TSM TDI bind/unbind IOCTLs for TEE-IO support

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Jun 05, 2025 at 05:41:17PM +0800, Xu Yilun wrote:

> No, this is not device side TDISP requirement. It is host side
> requirement to fix DMA silent drop issue. TDX enforces CPU S2 PT share
> with IOMMU S2 PT (does ARM do the same?), so unmap CPU S2 PT in KVM equals
> unmap IOMMU S2 PT.
> 
> If we allow IOMMU S2 PT unmapped when TDI is running, host could fool
> guest by just unmap some PT entry and suppress the fault event. Guest
> thought a DMA writting is successful but it is not and may cause
> data integrity issue.

So, TDX prevents *any* unmap, even of normal memory, from the S2 while
a guest is running?  Seems extreme?

MMIO isn't special, if you have a rule like that for such a security
reason it should cover all of the S2.

> This is not a TDX specific problem, but different vendors has different
> mechanisms for this. For TDX, firmware fails the MMIO unmap for S2. For
> AMD, will trigger some HW protection called "ASID fence" [1]. Not sure
> how ARM handles this?

This seems even more extreme, if the guest gets a bad DMA address into
the device then the entire device gets killed? No chance to debug it?

Jason