[xen staging] xen/arm: Introduce CONFIG_PARTIAL_EMULATION and "partial-emulation" cmd option

[patchbot@xxxxxxx]

commit bd06cf74fccc065ea67e9dd8c31e70abad1f0507
Author:     Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
AuthorDate: Thu Mar 7 12:39:41 2024 +0000
Commit:     Julien Grall <jgrall@xxxxxxxxxx>
CommitDate: Thu Mar 14 13:04:25 2024 +0000

    xen/arm: Introduce CONFIG_PARTIAL_EMULATION and "partial-emulation" cmd 
option
    
    There can be situations when the registers cannot be emulated to their full
    functionality. This can be due to the complexity involved. In such cases, 
one
    can emulate those registers as RAZ/WI for example. We call them as partial
    emulation.
    
    Some registers are non-optional and as such there is nothing preventing an 
OS
    from accessing them.
    Instead of injecting undefined exception (which might result in crashing a
    guest), one may want to prefer a partial emulation to let the guest running
    (in some cases accepting the fact that it might result in unwanted 
behavior).
    
    A suitable example of this (as seen in subsequent patches) is emulation of
    DBGDTRTX_EL0 (on Arm64) and DBGDTRTXINT(on Arm32). These non-optional
    registers can be emulated as RAZ/WI and they can be enclosed within
    CONFIG_PARTIAL_EMULATION.
    
    Further, "partial-emulation" command line option allows us to
    enable/disable partial emulation at run time. While CONFIG_PARTIAL_EMULATION
    enables support for partial emulation at compile time (i.e. adds code for
    partial emulation), this option may be enabled or disabled by Yocto or other
    build systems. However if the build system turns this option on, users
    can use scripts like Imagebuilder to generate uboot-script which will append
    "partial-emulation=true" to xen command line to turn on the partial 
emulation.
    Thus, it helps to avoid rebuilding xen.
    
    By default, "CONFIG_PARTIAL_EMULATION=y" and "partial-emulation=false".
    This is done so that Xen supports partial emulation. However, customers are
    fully aware when they enable partial emulation. It's important to note that
    enabling such support might result in unwanted/non-spec compliant behavior.
    
    Added a note in SUPPORT.md to clarify the security support for partial
    emulation.
    
    Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
    Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
    Acked-by: Julien Grall <jgrall@xxxxxxxxxx>
---
 SUPPORT.md                        |  9 +++++++++
 docs/misc/xen-command-line.pandoc | 11 +++++++++++
 xen/arch/arm/Kconfig              |  9 +++++++++
 xen/arch/arm/include/asm/traps.h  |  6 ++++++
 xen/arch/arm/traps.c              |  9 +++++++++
 5 files changed, 44 insertions(+)

diff --git a/SUPPORT.md b/SUPPORT.md
index 510bb02190..ad74f5a329 100644
--- a/SUPPORT.md
+++ b/SUPPORT.md
@@ -102,6 +102,15 @@ Extension to the GICv3 interrupt controller to support MSI.
 
     Status: Experimental
 
+### ARM/Partial Emulation
+
+Enable partial emulation of registers, otherwise considered unimplemented,
+that would normally trigger a fault injection.
+
+    Status: Supported, with caveats
+
+Only the following system registers are security supported:
+
 ### ARM Scalable Vector Extension (SVE/SVE2)
 
 Arm64 domains can use Scalable Vector Extension (SVE/SVE2).
diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index 04f1415fc3..14cafb462a 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -1942,6 +1942,17 @@ This option is ignored in **pv-shim** mode.
 
 > Default: `on`
 
+### partial-emulation (arm)
+> `= <boolean>`
+
+> Default: `false`
+
+Flag to enable or disable partial emulation of system/coprocessor registers.
+Only effective if CONFIG_PARTIAL_EMULATION is enabled.
+
+**WARNING: Enabling this option might result in unwanted/non-spec compliant
+behavior.**
+
 ### pci
     = List of [ serr=<bool>, perr=<bool> ]
 
diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
index 40f834bb71..f8139a773a 100644
--- a/xen/arch/arm/Kconfig
+++ b/xen/arch/arm/Kconfig
@@ -234,6 +234,15 @@ config STATIC_EVTCHN
          This option enables establishing static event channel communication
          between domains on a dom0less system (domU-domU as well as domU-dom0).
 
+config PARTIAL_EMULATION
+       bool "Enable partial emulation of system/coprocessor registers"
+       default y
+       help
+         This option enables partial emulation of registers to prevent possible
+         guests crashing when accessing registers which are not optional but 
have
+         not been emulated to their complete functionality. Enabling this might
+         result in unwanted/non-spec compliant behavior.
+
 endmenu
 
 menu "ARM errata workaround via the alternative framework"
diff --git a/xen/arch/arm/include/asm/traps.h b/xen/arch/arm/include/asm/traps.h
index 883dae368e..9a60dbf70e 100644
--- a/xen/arch/arm/include/asm/traps.h
+++ b/xen/arch/arm/include/asm/traps.h
@@ -10,6 +10,12 @@
 # include <asm/arm64/traps.h>
 #endif
 
+#ifdef CONFIG_PARTIAL_EMULATION
+extern bool partial_emulation;
+#else
+#define partial_emulation false
+#endif
+
 /*
  * GUEST_BUG_ON is intended for checking that the guest state has not been
  * corrupted in hardware and/or that the hardware behaves as we
diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index 8ddca643d4..9cffe7f790 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -43,6 +43,15 @@
 #include <asm/vgic.h>
 #include <asm/vtimer.h>
 
+/*
+ * partial_emulation: If true, partial emulation for system/coprocessor
+ * registers will be enabled.
+ */
+#ifdef CONFIG_PARTIAL_EMULATION
+bool __ro_after_init partial_emulation = false;
+boolean_param("partial-emulation", partial_emulation);
+#endif
+
 /* The base of the stack must always be double-word aligned, which means
  * that both the kernel half of struct cpu_user_regs (which is pushed in
  * entry.S) and struct cpu_info (which lives at the bottom of a Xen
--
generated by git-patchbot for /home/xen/git/xen.git#staging