[PATCH v2] LoongArch: expose LAMCAS existence via cpuinfo and hwcap

From: Rong Bao

Date: Sun Sep 06 2026 - 04:04:18 EST

Currently, neither /proc/cpuinfo nor hwcap exposes the existence of the
LAMCAS extension, which is part of LA64v1.1. For example, on a Loongson
3B6000, /proc/cpuinfo is as follows:

processor : 23
package : 0
core : 11
global_id : 31
CPU Family : Loongson-64bit
Model Name : Loongson-3B6000
PRID : LA664 (0014d011)
CPU Revision : 0x11
FPU Revision : 0x01
CPU MHz : 2200.00
BogoMIPS : 4400.00
TLB Entries : 2112
Address Sizes : 48 bits physical, 48 bits virtual
ISA : loongarch32r loongarch32s loongarch64
Features : cpucfg lam lam_bh scq ual fpu lsx lasx
crc32 complex crypto lspw lvz lbt_x86 lbt_arm lbt_mips
Hardware Watchpoint : yes, iwatch count: 8, dwatch count: 4

... while the CPUCFG 2 instruction reads "0x7e7ccfcf", whose bit 28 is 1,
indicating the actual presence of LAMCAS.

This hinders user-space inspection and detection of LAMCAS. For example,
in SpiderMonkey, we need to switch to larchintrin.h and raw __cpucfg()
to manually perform the bit extraction. Also, a curious user reading
/proc/cpuinfo might be puzzled by the lack (of display) of LAMCAS.

Expose the existence of LAMCAS via cpuinfo and hwcap. The definition
point of the new LOONGARCH_CPU_LAMCAS flag is chosen to match the
practices in commit 1dd3e8a8eeb4059fb34b07578362380cf35b7ed5 ("LoongArch:
Define instruction formats for AM{SWAP/ADD}.{B/H} and DBAR").

After this patch, /proc/cpuinfo looks like the following:

processor : 23
package : 0
core : 11
global_id : 31
CPU Family : Loongson-64bit
Model Name : Loongson-3B6000
PRID : LA664 (0014d011)
CPU Revision : 0x11
FPU Revision : 0x01
CPU MHz : 2200.00
BogoMIPS : 4400.00
TLB Entries : 2112
Address Sizes : 48 bits physical, 48 bits virtual
ISA : loongarch32r loongarch32s loongarch64
Features : cpucfg lam lam_bh lamcas scq ual fpu lsx
lasx crc32 complex crypto lspw lvz lbt_x86 lbt_arm lbt_mips
Hardware Watchpoint : yes, iwatch count: 8, dwatch count: 4

Note the addition of "lamcas" to the "Features" row.

Link: https://bugzil.la/2069431 ("[loong64] Use LA64v1.1 LAMCAS
extension to simplify atomic cmpxchg operations")
Signed-off-by: Rong Bao <rong.bao@xxxxxxxxxxxx>
---
v1 -> v2:
- Addressed review comments and correctly re-stylized "LoongArch" in
subject.

v1: https://lore.kernel.org/loongarch/20260905032010.4962-1-rong.bao@xxxxxxxxxxxx/

arch/loongarch/include/asm/cpu-features.h | 1 +
arch/loongarch/include/asm/cpu.h | 64 ++++++++++++-----------
arch/loongarch/include/uapi/asm/hwcap.h | 1 +
arch/loongarch/kernel/cpu-probe.c | 4 ++
arch/loongarch/kernel/proc.c | 2 +
5 files changed, 41 insertions(+), 31 deletions(-)

diff --git a/arch/loongarch/include/asm/cpu-features.h b/arch/loongarch/include/asm/cpu-features.h
index 62059c5551b9cdcc63c26033f8ccb44d63b18482..19cb0ccd0fb62bbbeb80a31bc447c101560bed32 100644
--- a/arch/loongarch/include/asm/cpu-features.h
+++ b/arch/loongarch/include/asm/cpu-features.h
@@ -36,6 +36,7 @@
#define cpu_has_cpucfg cpu_opt(LOONGARCH_CPU_CPUCFG)
#define cpu_has_lam cpu_opt(LOONGARCH_CPU_LAM)
#define cpu_has_lam_bh cpu_opt(LOONGARCH_CPU_LAM_BH)
+#define cpu_has_lamcas cpu_opt(LOONGARCH_CPU_LAMCAS)
#define cpu_has_scq cpu_opt(LOONGARCH_CPU_SCQ)
#define cpu_has_ual cpu_opt(LOONGARCH_CPU_UAL)
#define cpu_has_fpu cpu_opt(LOONGARCH_CPU_FPU)
diff --git a/arch/loongarch/include/asm/cpu.h b/arch/loongarch/include/asm/cpu.h
index 91b96938861eb9f81882b128ab0bd0a925ce0dbb..334ca9d69ec4a649848133d0a81555bfc37fbf9d 100644
--- a/arch/loongarch/include/asm/cpu.h
+++ b/arch/loongarch/include/asm/cpu.h
@@ -96,41 +96,43 @@ static inline char *id_to_core_name(unsigned int id)
#define CPU_FEATURE_CPUCFG 0 /* CPU has CPUCFG */
#define CPU_FEATURE_LAM 1 /* CPU has Atomic instructions */
#define CPU_FEATURE_LAM_BH 2 /* CPU has AM{SWAP/ADD}[_DB].{B/H} instructions */
-#define CPU_FEATURE_SCQ 3 /* CPU has SC.Q instruction */
-#define CPU_FEATURE_UAL 4 /* CPU supports unaligned access */
-#define CPU_FEATURE_FPU 5 /* CPU has FPU */
-#define CPU_FEATURE_LSX 6 /* CPU has LSX (128-bit SIMD) */
-#define CPU_FEATURE_LASX 7 /* CPU has LASX (256-bit SIMD) */
-#define CPU_FEATURE_CRC32 8 /* CPU has CRC32 instructions */
-#define CPU_FEATURE_COMPLEX 9 /* CPU has Complex instructions */
-#define CPU_FEATURE_CRYPTO 10 /* CPU has Crypto instructions */
-#define CPU_FEATURE_LVZ 11 /* CPU has Virtualization extension */
-#define CPU_FEATURE_LBT_X86 12 /* CPU has X86 Binary Translation */
-#define CPU_FEATURE_LBT_ARM 13 /* CPU has ARM Binary Translation */
-#define CPU_FEATURE_LBT_MIPS 14 /* CPU has MIPS Binary Translation */
-#define CPU_FEATURE_TLB 15 /* CPU has TLB */
-#define CPU_FEATURE_CSR 16 /* CPU has CSR */
-#define CPU_FEATURE_IOCSR 17 /* CPU has IOCSR */
-#define CPU_FEATURE_WATCH 18 /* CPU has watchpoint registers */
-#define CPU_FEATURE_VINT 19 /* CPU has vectored interrupts */
-#define CPU_FEATURE_CSRIPI 20 /* CPU has CSR-IPI */
-#define CPU_FEATURE_EXTIOI 21 /* CPU has EXT-IOI */
-#define CPU_FEATURE_PREFETCH 22 /* CPU has prefetch instructions */
-#define CPU_FEATURE_PMP 23 /* CPU has perfermance counter */
-#define CPU_FEATURE_SCALEFREQ 24 /* CPU supports cpufreq scaling */
-#define CPU_FEATURE_FLATMODE 25 /* CPU has flat mode */
-#define CPU_FEATURE_EIODECODE 26 /* CPU has EXTIOI interrupt pin decode mode */
-#define CPU_FEATURE_GUESTID 27 /* CPU has GuestID feature */
-#define CPU_FEATURE_HYPERVISOR 28 /* CPU has hypervisor (running in VM) */
-#define CPU_FEATURE_PTW 29 /* CPU has hardware page table walker */
-#define CPU_FEATURE_LSPW 30 /* CPU has LSPW (lddir/ldpte instructions) */
-#define CPU_FEATURE_MSGINT 31 /* CPU has MSG interrupt */
-#define CPU_FEATURE_AVECINT 32 /* CPU has AVEC interrupt */
-#define CPU_FEATURE_REDIRECTINT 33 /* CPU has interrupt remapping */
+#define CPU_FEATURE_LAMCAS 3 /* CPU has AMCAS[_DB].{B/H/W/D} instructions */
+#define CPU_FEATURE_SCQ 4 /* CPU has SC.Q instruction */
+#define CPU_FEATURE_UAL 5 /* CPU supports unaligned access */
+#define CPU_FEATURE_FPU 6 /* CPU has FPU */
+#define CPU_FEATURE_LSX 7 /* CPU has LSX (128-bit SIMD) */
+#define CPU_FEATURE_LASX 8 /* CPU has LASX (256-bit SIMD) */
+#define CPU_FEATURE_CRC32 9 /* CPU has CRC32 instructions */
+#define CPU_FEATURE_COMPLEX 10 /* CPU has Complex instructions */
+#define CPU_FEATURE_CRYPTO 11 /* CPU has Crypto instructions */
+#define CPU_FEATURE_LVZ 12 /* CPU has Virtualization extension */
+#define CPU_FEATURE_LBT_X86 13 /* CPU has X86 Binary Translation */
+#define CPU_FEATURE_LBT_ARM 14 /* CPU has ARM Binary Translation */
+#define CPU_FEATURE_LBT_MIPS 15 /* CPU has MIPS Binary Translation */
+#define CPU_FEATURE_TLB 16 /* CPU has TLB */
+#define CPU_FEATURE_CSR 17 /* CPU has CSR */
+#define CPU_FEATURE_IOCSR 18 /* CPU has IOCSR */
+#define CPU_FEATURE_WATCH 19 /* CPU has watchpoint registers */
+#define CPU_FEATURE_VINT 20 /* CPU has vectored interrupts */
+#define CPU_FEATURE_CSRIPI 21 /* CPU has CSR-IPI */
+#define CPU_FEATURE_EXTIOI 22 /* CPU has EXT-IOI */
+#define CPU_FEATURE_PREFETCH 23 /* CPU has prefetch instructions */
+#define CPU_FEATURE_PMP 24 /* CPU has perfermance counter */
+#define CPU_FEATURE_SCALEFREQ 25 /* CPU supports cpufreq scaling */
+#define CPU_FEATURE_FLATMODE 26 /* CPU has flat mode */
+#define CPU_FEATURE_EIODECODE 27 /* CPU has EXTIOI interrupt pin decode mode */
+#define CPU_FEATURE_GUESTID 28 /* CPU has GuestID feature */
+#define CPU_FEATURE_HYPERVISOR 29 /* CPU has hypervisor (running in VM) */
+#define CPU_FEATURE_PTW 30 /* CPU has hardware page table walker */
+#define CPU_FEATURE_LSPW 31 /* CPU has LSPW (lddir/ldpte instructions) */
+#define CPU_FEATURE_MSGINT 32 /* CPU has MSG interrupt */
+#define CPU_FEATURE_AVECINT 33 /* CPU has AVEC interrupt */
+#define CPU_FEATURE_REDIRECTINT 34 /* CPU has interrupt remapping */

#define LOONGARCH_CPU_CPUCFG BIT_ULL(CPU_FEATURE_CPUCFG)
#define LOONGARCH_CPU_LAM BIT_ULL(CPU_FEATURE_LAM)
#define LOONGARCH_CPU_LAM_BH BIT_ULL(CPU_FEATURE_LAM_BH)
+#define LOONGARCH_CPU_LAMCAS BIT_ULL(CPU_FEATURE_LAMCAS)
#define LOONGARCH_CPU_SCQ BIT_ULL(CPU_FEATURE_SCQ)
#define LOONGARCH_CPU_UAL BIT_ULL(CPU_FEATURE_UAL)
#define LOONGARCH_CPU_FPU BIT_ULL(CPU_FEATURE_FPU)
diff --git a/arch/loongarch/include/uapi/asm/hwcap.h b/arch/loongarch/include/uapi/asm/hwcap.h
index 90e96113ba51003cd4dde5c7c79744f5e0755a03..d354787b937c3ae9eb8c3d58a4e7e96bd066f6fd 100644
--- a/arch/loongarch/include/uapi/asm/hwcap.h
+++ b/arch/loongarch/include/uapi/asm/hwcap.h
@@ -20,5 +20,6 @@
#define HWCAP_LOONGARCH_LSPW (1 << 14)
#define HWCAP_LOONGARCH_SCQ (1 << 15)
#define HWCAP_LOONGARCH_LAM_BH (1 << 16)
+#define HWCAP_LOONGARCH_LAMCAS (1 << 17)

#endif /* _UAPI_ASM_HWCAP_H */
diff --git a/arch/loongarch/kernel/cpu-probe.c b/arch/loongarch/kernel/cpu-probe.c
index 74d31f260dfd889e66a1f55c5bcc47a3657d101f..93176e4d017b4d8fc82c817a7064af0656133c15 100644
--- a/arch/loongarch/kernel/cpu-probe.c
+++ b/arch/loongarch/kernel/cpu-probe.c
@@ -182,6 +182,10 @@ static void cpu_probe_common(struct cpuinfo_loongarch *c)
c->options |= LOONGARCH_CPU_LAM_BH;
elf_hwcap |= HWCAP_LOONGARCH_LAM_BH;
}
+ if (config & CPUCFG2_LAMCAS) {
+ c->options |= LOONGARCH_CPU_LAMCAS;
+ elf_hwcap |= HWCAP_LOONGARCH_LAMCAS;
+ }
if (config & CPUCFG2_SCQ) {
c->options |= LOONGARCH_CPU_SCQ;
elf_hwcap |= HWCAP_LOONGARCH_SCQ;
diff --git a/arch/loongarch/kernel/proc.c b/arch/loongarch/kernel/proc.c
index d4ce5b585453700caf2187c3dcc8f4f3cc8a9226..f86be83ad6f134c5793667fa6dbe2bf82fd58a2e 100644
--- a/arch/loongarch/kernel/proc.c
+++ b/arch/loongarch/kernel/proc.c
@@ -66,6 +66,8 @@ static int show_cpuinfo(struct seq_file *m, void *v)
seq_puts(m, " lam");
if (cpu_has_lam_bh)
seq_puts(m, " lam_bh");
+ if (cpu_has_lamcas)
+ seq_puts(m, " lamcas");
if (cpu_has_scq)
seq_puts(m, " scq");
if (cpu_has_ual)

base-commit: 654ae5d73c05bd2943d65636ce6cd0aa46e62f18
--
2.55.0