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authorMark Rutland <mark.rutland@arm.com>2022-09-12 17:22:09 +0100
committerCatalin Marinas <catalin.marinas@arm.com>2022-09-16 17:15:03 +0100
commit21fb26bfb01ffe0d9a9a967ffe061092128bbffe (patch)
treebb2fb072aa3f830ec93c96c6173af34406e73566 /arch/arm64/include/asm/cpufeature.h
parent4c0bd995d73ed8897650095c7892b132a0bd66a4 (diff)
arm64: alternatives: add alternative_has_feature_*()
Currrently we use a mixture of alternative sequences and static branches to handle features detected at boot time. For ease of maintenance we generally prefer to use static branches in C code, but this has a few downsides: * Each static branch has metadata in the __jump_table section, which is not discarded after features are finalized. This wastes some space, and slows down the patching of other static branches. * The static branches are patched at a different point in time from the alternatives, so changes are not atomic. This leaves a transient period where there could be a mismatch between the behaviour of alternatives and static branches, which could be problematic for some features (e.g. pseudo-NMI). * More (instrumentable) kernel code is executed to patch each static branch, which can be risky when patching certain features (e.g. irqflags management for pseudo-NMI). * When CONFIG_JUMP_LABEL=n, static branches are turned into a load of a flag and a conditional branch. This means it isn't safe to use such static branches in an alternative address space (e.g. the NVHE/PKVM hyp code), where the generated address isn't safe to acccess. To deal with these issues, this patch introduces new alternative_has_feature_*() helpers, which work like static branches but are patched using alternatives. This ensures the patching is performed at the same time as other alternative patching, allows the metadata to be freed after patching, and is safe for use in alternative address spaces. Note that all supported toolchains have asm goto support, and since commit: a0a12c3ed057af57 ("asm goto: eradicate CC_HAS_ASM_GOTO)" ... the CC_HAS_ASM_GOTO Kconfig symbol has been removed, so no feature check is necessary, and we can always make use of asm goto. Additionally, note that: * This has no impact on cpus_have_cap(), which is a dynamic check. * This has no functional impact on cpus_have_const_cap(). The branches are patched slightly later than before this patch, but these branches are not reachable until caps have been finalised. * It is now invalid to use cpus_have_final_cap() in the window between feature detection and patching. All existing uses are only expected after patching anyway, so this should not be a problem. * The LSE atomics will now be enabled during alternatives patching rather than immediately before. As the LL/SC an LSE atomics are functionally equivalent this should not be problematic. When building defconfig with GCC 12.1.0, the resulting Image is 64KiB smaller: | % ls -al Image-* | -rw-r--r-- 1 mark mark 37108224 Aug 23 09:56 Image-after | -rw-r--r-- 1 mark mark 37173760 Aug 23 09:54 Image-before According to bloat-o-meter.pl: | add/remove: 44/34 grow/shrink: 602/1294 up/down: 39692/-61108 (-21416) | Function old new delta | [...] | Total: Before=16618336, After=16596920, chg -0.13% | add/remove: 0/2 grow/shrink: 0/0 up/down: 0/-1296 (-1296) | Data old new delta | arm64_const_caps_ready 16 - -16 | cpu_hwcap_keys 1280 - -1280 | Total: Before=8987120, After=8985824, chg -0.01% | add/remove: 0/0 grow/shrink: 0/0 up/down: 0/0 (0) | RO Data old new delta | Total: Before=18408, After=18408, chg +0.00% Signed-off-by: Mark Rutland <mark.rutland@arm.com> Cc: Ard Biesheuvel <ardb@kernel.org> Cc: James Morse <james.morse@arm.com> Cc: Joey Gouly <joey.gouly@arm.com> Cc: Marc Zyngier <maz@kernel.org> Cc: Will Deacon <will@kernel.org> Reviewed-by: Ard Biesheuvel <ardb@kernel.org> Link: https://lore.kernel.org/r/20220912162210.3626215-8-mark.rutland@arm.com Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Diffstat (limited to 'arch/arm64/include/asm/cpufeature.h')
-rw-r--r--arch/arm64/include/asm/cpufeature.h7
1 files changed, 3 insertions, 4 deletions
diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index f5852ad6e884..ba8a47433b5a 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -6,6 +6,7 @@
#ifndef __ASM_CPUFEATURE_H
#define __ASM_CPUFEATURE_H
+#include <asm/alternative-macros.h>
#include <asm/cpucaps.h>
#include <asm/cputype.h>
#include <asm/hwcap.h>
@@ -419,8 +420,6 @@ static __always_inline bool is_hyp_code(void)
}
extern DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
-extern struct static_key_false cpu_hwcap_keys[ARM64_NCAPS];
-extern struct static_key_false arm64_const_caps_ready;
extern DECLARE_BITMAP(boot_capabilities, ARM64_NCAPS);
@@ -438,7 +437,7 @@ unsigned long cpu_get_elf_hwcap2(void);
static __always_inline bool system_capabilities_finalized(void)
{
- return static_branch_likely(&arm64_const_caps_ready);
+ return alternative_has_feature_likely(ARM64_ALWAYS_SYSTEM);
}
/*
@@ -465,7 +464,7 @@ static __always_inline bool __cpus_have_const_cap(int num)
{
if (num >= ARM64_NCAPS)
return false;
- return static_branch_unlikely(&cpu_hwcap_keys[num]);
+ return alternative_has_feature_unlikely(num);
}
/*