x86/spec_ctrl: Introduce a new spec-ctrl= command line argument to replace bti=

In hindsight, the options for `bti=` aren't as flexible or useful as expected
(including several options which don't appear to behave as intended).
Changing the behaviour of an existing option is problematic for compatibility,
so introduce a new `spec-ctrl=` in the hopes that we can do better.

One common way of deploying Xen is with a single PV dom0 and all domUs being
HVM domains.  In such a setup, an administrator who has weighed up the risks
may wish to forgo protection against malicious PV domains, to reduce the
overall performance hit.  To cater for this usecase, `spec-ctrl=no-pv` will
disable all speculative protection for PV domains, while leaving all
speculative protection for HVM domains intact.

For coding clarity as much as anything else, the suboptions are grouped by
logical area; those which affect the alternatives blocks, and those which
affect Xen's in-hypervisor settings.  See the xen-command-line.markdown for
full details of the new options.

While changing the command line options, take the time to change how the data
is reported to the user.  The three DEBUG printks are upgraded to unilateral,
as they are all relevant pieces of information, and the old "mitigations:"
line is split in the two logical areas described above.

Sample output from booting with `spec-ctrl=no-pv` looks like:

  (XEN) Speculative mitigation facilities:
  (XEN)   Hardware features: IBRS/IBPB STIBP IBPB
  (XEN)   Compiled-in support: INDIRECT_THUNK
  (XEN)   Xen settings: BTI-Thunk RETPOLINE, SPEC_CTRL: IBRS-, Other: IBPB
  (XEN)   Support for VMs: PV: None, HVM: MSR_SPEC_CTRL RSB
  (XEN)   XPTI (64-bit PV only): Dom0 enabled, DomU enabled

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 3352afc26c
master date: 2018-05-16 12:19:10 +0100

docs/misc/xen-command-line.markdown

@@ -247,6 +247,9 @@ the NMI watchdog is also enabled.
 ### bti (x86)
 > `= List of [ thunk=retpoline|lfence|jmp, ibrs=<bool>, ibpb=<bool>, rsb_{vmexit,native}=<bool> ]`
 
+**WARNING: This command line option is deprecated, and superseded by
+_spec-ctrl=_ - using both options in combination is undefined.**
+
 Branch Target Injection controls.  By default, Xen will pick the most
 appropriate BTI mitigations based on compiled in support, loaded microcode,
 and hardware details.
@@ -1394,6 +1397,52 @@ enforces the maximum theoretically necessary timeout of 670ms. Any number
 is being interpreted as a custom timeout in milliseconds. Zero or boolean
 false disable the quirk workaround, which is also the default.
 
+### spec-ctrl (x86)
+> `= List of [ <bool>, xen=<bool>, {pv,hvm,msr-sc,rsb}=<bool>,
+>              bti-thunk=retpoline|lfence|jmp, {ibrs,ibpb}=<bool> ]`
+
+Controls for speculative execution sidechannel mitigations.  By default, Xen
+will pick the most appropriate mitigations based on compiled in support,
+loaded microcode, and hardware details, and will virtualise appropriate
+mitigations for guests to use.
+
+**WARNING: Any use of this option may interfere with heuristics.  Use with
+extreme care.**
+
+An overall boolean value, `spec-ctrl=no`, can be specified to turn off all
+mitigations, including pieces of infrastructure used to virtualise certain
+mitigation features for guests.  Alternatively, a slightly more restricted
+`spec-ctrl=no-xen` can be used to turn off all of Xen's mitigations, while
+leaving the virtualisation support in place for guests to use.  Use of a
+positive boolean value for either of these options is invalid.
+
+The booleans `pv=`, `hvm=`, `msr-sc=` and `rsb=` offer fine grained control
+over the alternative blocks used by Xen.  These impact Xen's ability to
+protect itself, and Xen's ability to virtualise support for guests to use.
+
+* `pv=` and `hvm=` offer control over all suboptions for PV and HVM guests
+  respectively.
+* `msr-sc=` offers control over Xen's support for manipulating MSR\_SPEC\_CTRL
+  on entry and exit.  These blocks are necessary to virtualise support for
+  guests and if disabled, guests will be unable to use IBRS/STIBP/etc.
+* `rsb=` offers control over whether to overwrite the Return Stack Buffer /
+  Return Address Stack on entry to Xen.
+
+If Xen was compiled with INDIRECT\_THUNK support, `bti-thunk=` can be used to
+select which of the thunks gets patched into the `__x86_indirect_thunk_%reg`
+locations.  The default thunk is `retpoline` (generally preferred for Intel
+hardware), with the alternatives being `jmp` (a `jmp *%reg` gadget, minimal
+overhead), and `lfence` (an `lfence; jmp *%reg` gadget, preferred for AMD).
+
+On hardware supporting IBRS (Indirect Branch Restricted Speculation), the
+`ibrs=` option can be used to force or prevent Xen using the feature itself.
+If Xen is not using IBRS itself, functionality is still set up so IBRS can be
+virtualised for guests.
+
+On hardware supporting IBPB (Indirect Branch Prediction Barrier), the `ibpb=`
+option can be used to force (the default) or prevent Xen from issuing branch
+prediction barriers on vcpu context switches.
+
 ### sync\_console
 > `= <boolean>`