[v4,32/38] arm64: Avoid cpus_have_const_cap() for ARM64_WORKAROUND_1542419

Commit Message

Mark Rutland Oct. 16, 2023, 10:24 a.m. UTC

We use cpus_have_const_cap() to check for ARM64_WORKAROUND_1542419 but
this is not necessary and cpus_have_final_cap() would be preferable.

For historical reasons, cpus_have_const_cap() is more complicated than
it needs to be. Before cpucaps are finalized, it will perform a bitmap
test of the system_cpucaps bitmap, and once cpucaps are finalized it
will use an alternative branch. This used to be necessary to handle some
race conditions in the window between cpucap detection and the
subsequent patching of alternatives and static branches, where different
branches could be out-of-sync with one another (or w.r.t. alternative
sequences). Now that we use alternative branches instead of static
branches, these are all patched atomically w.r.t. one another, and there
are only a handful of cases that need special care in the window between
cpucap detection and alternative patching.

Due to the above, it would be nice to remove cpus_have_const_cap(), and
migrate callers over to alternative_has_cap_*(), cpus_have_final_cap(),
or cpus_have_cap() depending on when their requirements. This will
remove redundant instructions and improve code generation, and will make
it easier to determine how each callsite will behave before, during, and
after alternative patching.

The ARM64_WORKAROUND_1542419 cpucap is detected and patched before any
userspace code can run, and the both __do_compat_cache_op() and
ctr_read_handler() are only reachable from exceptions taken from
userspace. Thus it is not necessary for either to use
cpus_have_const_cap(), and cpus_have_final_cap() is equivalent.

This patch replaces the use of cpus_have_const_cap() with
cpus_have_final_cap(), which will avoid generating code to test the
system_cpucaps bitmap and should be better for all subsequent calls at
runtime. Using cpus_have_final_cap() clearly documents that we do not
expect this code to run before cpucaps are finalized, and will make it
easier to spot issues if code is changed in future to allow these
functions to be reached earlier.

Signed-off-by: Mark Rutland <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Suzuki K Poulose <[e-mail]>
Cc: Will Deacon <[e-mail]>
---
 arch/arm64/kernel/sys_compat.c | 2 +-
 arch/arm64/kernel/traps.c      | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

Patch

diff --git a/arch/arm64/kernel/sys_compat.c b/arch/arm64/kernel/sys_compat.c
index df14336c3a29c..4a609e9b65de0 100644
--- a/arch/arm64/kernel/sys_compat.c
+++ b/arch/arm64/kernel/sys_compat.c
@@ -31,7 +31,7 @@  __do_compat_cache_op(unsigned long start, unsigned long end)
 		if (fatal_signal_pending(current))
 			return 0;
 
-		if (cpus_have_const_cap(ARM64_WORKAROUND_1542419)) {
+		if (cpus_have_final_cap(ARM64_WORKAROUND_1542419)) {
 			/*
 			 * The workaround requires an inner-shareable tlbi.
 			 * We pick the reserved-ASID to minimise the impact.
diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 8b70759cdbb90..9eba6cdd70382 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -631,7 +631,7 @@  static void ctr_read_handler(unsigned long esr, struct pt_regs *regs)
 	int rt = ESR_ELx_SYS64_ISS_RT(esr);
 	unsigned long val = arm64_ftr_reg_user_value(&arm64_ftr_reg_ctrel0);
 
-	if (cpus_have_const_cap(ARM64_WORKAROUND_1542419)) {
+	if (cpus_have_final_cap(ARM64_WORKAROUND_1542419)) {
 		/* Hide DIC so that we can trap the unnecessary maintenance...*/
 		val &= ~BIT(CTR_EL0_DIC_SHIFT);