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

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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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@amd.com>
> > > 
> > > 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).

References:
https://lists.xenproject.org/archives/html/xen-devel/2022-06/msg01660.html
https://github.com/QubesOS/qubes-issues/issues/6923

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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