x86/spec-ctrl: Add spec-ctrl=unpriv-mmio

Per Xen's support statement, PCI passthrough should be to trusted domains
because the overall system security depends on factors outside of Xen's
control.

As such, Xen, in a supported configuration, is not vulnerable to DRPW/SBDR.

However, users who have risk assessed their configuration may be happy with
the risk of DoS, but unhappy with the risk of cross-domain data leakage.  Such
users should enable this option.

On CPUs vulnerable to MDS, the existing mitigations are the best we can do to
mitigate MMIO cross-domain data leakage.

On CPUs fixed to MDS but vulnerable MMIO stale data leakage, this option:

 * On CPUs susceptible to FBSDP, mitigates cross-domain fill buffer leakage
   using FB_CLEAR.
 * On CPUs susceptible to SBDR, mitigates RNG data recovery by engaging the
   srb-lock, previously used to mitigate SRBDS.

Both mitigations require microcode from IPU 2022.1, May 2022.

This is part of XSA-404.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

docs/misc/xen-command-line.pandoc

@@ -2259,7 +2259,7 @@ By default SSBD will be mitigated at runtime (i.e `ssbd=runtime`).
 ### spec-ctrl (x86)
 > `= List of [ <bool>, xen=<bool>, {pv,hvm,msr-sc,rsb,md-clear}=<bool>,
 >              bti-thunk=retpoline|lfence|jmp, {ibrs,ibpb,ssbd,eager-fpu,
->              l1d-flush,branch-harden,srb-lock}=<bool> ]`
+>              l1d-flush,branch-harden,srb-lock,unpriv-mmio}=<bool> ]`
 
 Controls for speculative execution sidechannel mitigations.  By default, Xen
 will pick the most appropriate mitigations based on compiled in support,
@@ -2338,8 +2338,16 @@ Xen will enable this mitigation.
 On hardware supporting SRBDS_CTRL, the `srb-lock=` option can be used to force
 or prevent Xen from protect the Special Register Buffer from leaking stale
 data. By default, Xen will enable this mitigation, except on parts where MDS
-is fixed and TAA is fixed/mitigated (in which case, there is believed to be no
-way for an attacker to obtain the stale data).
+is fixed and TAA is fixed/mitigated and there are no unprivileged MMIO
+mappings (in which case, there is believed to be no way for an attacker to
+obtain stale data).
+
+The `unpriv-mmio=` boolean indicates whether the system has (or will have)
+less than fully privileged domains granted access to MMIO devices.  By
+default, this option is disabled.  If enabled, Xen will use the `FB_CLEAR`
+and/or `SRBDS_CTRL` functionality available in the Intel May 2022 microcode
+release to mitigate cross-domain leakage of data via the MMIO Stale Data
+vulnerabilities.
 
 ### sync_console
 > `= <boolean>`