Re: [PATCH v4 11/15] pci: Add pci_iomap_shared{,_range}

From: Michael S. Tsirkin
Date: Tue Aug 24 2021 - 05:12:17 EST

On Mon, Aug 23, 2021 at 05:30:54PM -0700, Kuppuswamy, Sathyanarayanan wrote:
> On 8/23/21 4:56 PM, Michael S. Tsirkin wrote:
> > > Add a new variant of pci_iomap for mapping all PCI resources
> > > of a devices as shared memory with a hypervisor in a confidential
> > > guest.
> > >
> > > Signed-off-by: Andi Kleen<ak@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Kuppuswamy Sathyanarayanan<sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> > I'm a bit puzzled by this part. So why should the guest*not* map
> > pci memory as shared? And if the answer is never (as it seems to be)
> > then why not just make regular pci_iomap DTRT?
> It is in the context of confidential guest (where VMM is un-trusted). So
> we don't want to make all PCI resource as shared. It should be allowed
> only for hardened drivers/devices.

I can't say this answers the question at all. PCI devices are part of
the VMM and so un-trusted. In particular PCI devices do not have
the key to decrypt memory. Therefore as far as I can see PCI resources
should not be encrypted. I conclude they all should be marked

If I'm wrong can you please give an example of a PCI resource
that is encrypted?