[PATCH v2] arm64: Add the arm64.no32bit_el0 command line option

[Andrea della Porta <andrea.porta@suse.com>]

Introducing the field 'el0' to the idreg-override for register
ID_AA64PFR0_EL1. This field is also aliased to the new kernel
command line option 'arm64.no32bit_el0' as a more recognizable
and mnemonic name to disable the execution of 32 bit userspace
applications (i.e. avoid Aarch32 execution state in EL0) from
kernel command line.

Changes in V2:
- fixed the order of appearance of arm64.no32bit_el0 entry in
  kernel-parameters.txt documentation.

Link: https://lore.kernel.org/r/ZVTleETzfFUchs77@apocalypse
Signed-off-by: Andrea della Porta <andrea.porta@suse.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 3 +++
 arch/arm64/kernel/idreg-override.c              | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 65731b060e3f..fa7cdf2f4f3a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -415,6 +415,9 @@
 	arcrimi=	[HW,NET] ARCnet - "RIM I" (entirely mem-mapped) cards
 			Format: <io>,<irq>,<nodeID>
 
+	arm64.no32bit_el0 [ARM64] Unconditionally disable the execution of
+			32 bit applications
+
 	arm64.nobti	[ARM64] Unconditionally disable Branch Target
 			Identification support
 
diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
index e30fd9e32ef3..642cda19e42d 100644
--- a/arch/arm64/kernel/idreg-override.c
+++ b/arch/arm64/kernel/idreg-override.c
@@ -86,6 +86,7 @@ static const struct ftr_set_desc pfr0 __prel64_initconst = {
 	.override	= &id_aa64pfr0_override,
 	.fields		= {
 	        FIELD("sve", ID_AA64PFR0_EL1_SVE_SHIFT, pfr0_sve_filter),
+		FIELD("el0", ID_AA64PFR0_EL1_EL0_SHIFT, NULL),
 		{}
 	},
 };
@@ -197,6 +198,7 @@ static const struct {
 	{ "arm64.nomops",		"id_aa64isar2.mops=0" },
 	{ "arm64.nomte",		"id_aa64pfr1.mte=0" },
 	{ "nokaslr",			"arm64_sw.nokaslr=1" },
+	{ "arm64.no32bit_el0",		"id_aa64pfr0.el0=1" },
 };
 
 static int __init parse_hexdigit(const char *p, u64 *v)
-- 
2.41.0