[PATCH v2 1/4] x86: provide an inverted Kconfig control for shim-exclusive mode

[Jan Beulich <jbeulich@xxxxxxxx>]

While we want certain things turned off in shim-exclusive mode, doing
so via "depends on !PV_SHIM_EXCLUSIVE" badly affects allyesconfig: Since
that will turn on PV_SHIM_EXCLUSIVE, other options will be turned off as
a result. Yet allyesconfig wants to enable as much of the functionality
as possible.

Retain PV_SHIM_EXCLUSIVE as a prompt-less option such that first of all
C code using it can remain as is. This isn't just for less code churn,
but also because I think that symbol is more logical to use in many
(all?) places.

Requested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
The new Kconfig control's name is up for improvement suggestions, but I
think it's already better than the originally thought of
FULL_HYPERVISOR.

Secondary Kconfig changes could be omitted; in all of the cases I wasn't
really sure whether do the adjustments. I think to avoid setting a bad
precedent we want to avoid "depends on !..." (and hence also the
functionally equivalent "if !..."), but any default settings or prompt
controls could also be left as they were (or could be done the other way
around in subsequent patches).

The Requested-by: isn't for what exactly is done here, but for the
underlying principle of avoiding the negative dependencies we've grown.

Outside of Arm-specific code we have two more negative "depends on":
COVERAGE requires !LIVEPATCH and SUPPRESS_DUPLICATE_SYMBOL_WARNINGS
requires !ENFORCE_UNIQUE_SYMBOLS. The latter could apparently be
switched to a choice (enforce, warn, don't warn), but then I'm not sure
how well choices play with allyesconfig (I guess the default setting is
used).
---
v2: New.

--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -103,7 +103,7 @@ config PV_LINEAR_PT
 
 config HVM
        bool "HVM support"
-       depends on !PV_SHIM_EXCLUSIVE
+       depends on UNCONSTRAINED
        default !PV_SHIM
        select COMPAT
        select IOREQ_SERVER
@@ -145,7 +145,7 @@ config XEN_IBT
 
 config SHADOW_PAGING
        bool "Shadow Paging"
-       default !PV_SHIM_EXCLUSIVE
+       default UNCONSTRAINED
        depends on PV || HVM
        ---help---
 
@@ -196,7 +196,7 @@ config HVM_FEP
 config TBOOT
        bool "Xen tboot support (UNSUPPORTED)"
        depends on UNSUPPORTED
-       default !PV_SHIM_EXCLUSIVE
+       default UNCONSTRAINED
        select CRYPTO
        ---help---
          Allows support for Trusted Boot using the Intel(R) Trusted Execution
@@ -276,17 +276,19 @@ config PV_SHIM
 
          If unsure, say Y.
 
-config PV_SHIM_EXCLUSIVE
-       bool "PV Shim Exclusive"
-       depends on PV_SHIM
-       ---help---
-         Build Xen in a way which unconditionally assumes PV_SHIM mode.  This
-         option is only intended for use when building a dedicated PV Shim
-         firmware, and will not function correctly in other scenarios.
+config UNCONSTRAINED
+       bool "do NOT build a functionality restricted hypervisor" if PV_SHIM
+       default y
+       help
+         Do NOT build Xen in a way which unconditionally assumes PV_SHIM mode.
 
-         If unsure, say N.
+         If unsure, say Y.
+
+config PV_SHIM_EXCLUSIVE
+       def_bool y
+       depends on !UNCONSTRAINED
 
-if !PV_SHIM_EXCLUSIVE
+if UNCONSTRAINED
 
 config HYPERV_GUEST
        bool "Hyper-V Guest"
--- a/xen/arch/x86/configs/pvshim_defconfig
+++ b/xen/arch/x86/configs/pvshim_defconfig
@@ -3,7 +3,7 @@ CONFIG_PV=y
 CONFIG_XEN_GUEST=y
 CONFIG_PVH_GUEST=y
 CONFIG_PV_SHIM=y
-CONFIG_PV_SHIM_EXCLUSIVE=y
+# CONFIG_UNCONSTRAINED is not set
 CONFIG_NR_CPUS=32
 CONFIG_EXPERT=y
 # Disable features not used by the PV shim
--- a/xen/drivers/video/Kconfig
+++ b/xen/drivers/video/Kconfig
@@ -3,10 +3,10 @@ config VIDEO
        bool
 
 config VGA
-       bool "VGA support" if !PV_SHIM_EXCLUSIVE
+       bool "VGA support" if UNCONSTRAINED
        select VIDEO
        depends on X86
-       default y if !PV_SHIM_EXCLUSIVE
+       default y if UNCONSTRAINED
        ---help---
          Enable VGA output for the Xen hypervisor.