Re: [PATCH] xen/x86: allow Dom0 PVH to call XENMEM_exchange

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, May 05, 2025 at 12:40:18PM +0200, Marek Marczykowski-Górecki wrote:
> On Mon, Apr 28, 2025 at 01:00:01PM -0700, Stefano Stabellini wrote:
> > On Mon, 28 Apr 2025, Jan Beulich wrote:
> > > On 25.04.2025 22:19, Stefano Stabellini wrote:
> > > > From: Xenia Ragiadakou <Xenia.Ragiadakou@xxxxxxx>
> > > > 
> > > > Dom0 PVH might need XENMEM_exchange when passing contiguous memory
> > > > addresses to firmware or co-processors not behind an IOMMU.
> > > 
> > > I definitely don't understand the firmware part: It's subject to the
> > > same transparent P2M translations as the rest of the VM; it's just
> > > another piece of software running there.
> > > 
> > > "Co-processors not behind an IOMMU" is also interesting; a more
> > > concrete scenario might be nice, yet I realize you may be limited in
> > > what you're allowed to say.
> > 
> > Sure. On AMD x86 platforms there is a co-processor called PSP running
> > TEE firmware. The PSP is not behind an IOMMU. Dom0 needs occasionally to
> > pass addresses to it.  See drivers/tee/amdtee/ and
> > include/linux/psp-tee.h in Linux.
> 
> We had (have?) similar issue with amdgpu (for integrated graphics) - it
> uses PSP for loading its firmware. With PV dom0 there is a workaround as
> dom0 kinda knows MFN. I haven't tried PVH dom0 on such system yet, but I
> expect troubles (BTW, hw1 aka zen2 gitlab runner has amdgpu, and it's
> the one I used for debugging this issue).

That's ugly, and problematic when used in conjunction with AMD-SEV.

I wonder if Xen could emulate/mediate some parts of the PSP for dom0
to use, while allowing Xen to be the sole owner of the device.  Having
both Xen and dom0 use it (for different purposes) seems like asking
for trouble.  But I also have no idea how complex the PSP interface
is, neither whether it would be feasible to emulate the
interfaces/registers needed for firmware loading.

Regards, Roger.