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Subject[PATCH 16/18] arm64: entry: Add fake CPU feature for mapping the kernel at EL0
Date
Allow explicit disabling of the entry trampoline on the kernel command
line by adding a fake CPU feature (ARM64_MAP_KERNEL_AT_EL0) that can
be used to apply alternative sequences to our entry code and avoid use
of the trampoline altogether.

Signed-off-by: Will Deacon <will.deacon@arm.com>
---
arch/arm64/include/asm/cpucaps.h | 3 ++-
arch/arm64/kernel/cpufeature.c | 11 +++++++++++
arch/arm64/kernel/entry.S | 6 +++++-
arch/arm64/mm/mmu.c | 7 +++++++
4 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
index 8da621627d7c..f61d85f76683 100644
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h
@@ -40,7 +40,8 @@
#define ARM64_WORKAROUND_858921 19
#define ARM64_WORKAROUND_CAVIUM_30115 20
#define ARM64_HAS_DCPOP 21
+#define ARM64_MAP_KERNEL_AT_EL0 22

-#define ARM64_NCAPS 22
+#define ARM64_NCAPS 23

#endif /* __ASM_CPUCAPS_H */
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 21e2c95d24e7..aa6b90de6591 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -796,6 +796,12 @@ static bool has_no_fpsimd(const struct arm64_cpu_capabilities *entry, int __unus
ID_AA64PFR0_FP_SHIFT) < 0;
}

+static bool map_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
+ int __unused)
+{
+ return arm64_kernel_mapped_at_el0();
+}
+
static const struct arm64_cpu_capabilities arm64_features[] = {
{
.desc = "GIC system register CPU interface",
@@ -883,6 +889,11 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
.matches = hyp_offset_low,
},
{
+ .capability = ARM64_MAP_KERNEL_AT_EL0,
+ .def_scope = SCOPE_SYSTEM,
+ .matches = map_kernel_at_el0,
+ },
+ {
/* FP/SIMD is not implemented */
.capability = ARM64_HAS_NO_FPSIMD,
.def_scope = SCOPE_SYSTEM,
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index a600879939ce..a74253defc5b 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -73,6 +73,7 @@
.macro kernel_ventry, el, label, regsize = 64
.align 7
#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
+alternative_if_not ARM64_MAP_KERNEL_AT_EL0
.if \el == 0
.if \regsize == 64
mrs x30, tpidrro_el0
@@ -80,6 +81,7 @@
mov x30, xzr
.endif
.endif
+alternative_else_nop_endif
#endif

sub sp, sp, #S_FRAME_SIZE
@@ -300,6 +302,7 @@ alternative_if ARM64_WORKAROUND_845719
alternative_else_nop_endif
#endif
#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
+alternative_if_not ARM64_MAP_KERNEL_AT_EL0
tramp_alias x30, tramp_exit_compat
b 4f
3:
@@ -308,6 +311,7 @@ alternative_else_nop_endif
tramp_alias x30, tramp_exit_native
4:
prfm plil1strm, [x30]
+alternative_else_nop_endif
#else
3:
#endif
@@ -332,7 +336,7 @@ alternative_else_nop_endif

#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
.if \el == 0
- br x30
+ alternative_insn "br x30", nop, ARM64_MAP_KERNEL_AT_EL0
.endif
#endif
eret
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 5ce5cb1249da..dab987f2912c 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -571,6 +571,13 @@ static int __init map_entry_trampoline(void)
return 0;
}
core_initcall(map_entry_trampoline);
+
+static int __init parse_nokaiser(char *__unused)
+{
+ static_branch_disable(&__unmap_kernel_at_el0);
+ return 0;
+}
+__setup("nokaiser", parse_nokaiser);
#else
static void __init add_tramp_vma(void) {}
#endif
--
2.1.4
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