Re: [PATCH 12/12] PCI/CMA: Grant guests exclusive control of authentication

[Lukas Wunner <lukas@wunner.de>]

On Mon, Oct 09, 2023 at 09:52:00PM +1100, Alexey Kardashevskiy wrote:
> On 29/9/23 03:32, Lukas Wunner wrote:
> > At any given time, only a single entity in a physical system may have
> > an SPDM connection to a device.  That's because the GET_VERSION request
> > (which begins an authentication sequence) resets "the connection and all
> > context associated with that connection" (SPDM 1.3.0 margin no 158).
> > 
> > Thus, when a device is passed through to a guest and the guest has
> > authenticated it, a subsequent authentication by the host would reset
> > the device's CMA-SPDM session behind the guest's back.
> > 
> > Prevent by letting the guest claim exclusive CMA ownership of the device
> > during passthrough.  Refuse CMA reauthentication on the host as long.
> > After passthrough has concluded, reauthenticate the device on the host.
[...]
> > --- a/drivers/pci/pci.h
> > +++ b/drivers/pci/pci.h
> > @@ -388,6 +388,7 @@ static inline bool pci_dev_is_disconnected(const struct pci_dev *dev)
> >   #define PCI_DEV_ADDED 0
> >   #define PCI_DPC_RECOVERED 1
> >   #define PCI_DPC_RECOVERING 2
> > +#define PCI_CMA_OWNED_BY_GUEST 3
> 
> In AMD SEV TIO, the PSP firmware creates an SPDM connection. What is the
> expected way of managing such ownership, a new priv_flags bit + api for it?

Right, I understand.  See this ongoing discussion in reply to the
cover letter:

https://lore.kernel.org/all/652030759e42d_ae7e72946@dwillia2-xfh.jf.intel.com.notmuch/

In short, we need a spec amendment to negotiate between platform and
OS which of the two controls the DOE instance supporting CMA-SPDM.

I think the OS is free to access any Extended Capabilities in
Config Space unless the platform doesn't grant it control over
them through _OSC.  Because the _OSC definition in the PCI
Firmware Spec was not amended for CMA-SPDM, it is legal for the
OS to assume control of CMA-SPDM, which is what this patch does.

Thanks,

Lukas