[PATCH v3 0/3] arm64: Support FEAT_LSFE (Large System Float Extension)

[PATCH v3 0/3] arm64: Support FEAT_LSFE (Large System Float Extension)

[Mark Brown]

FEAT_LSFE is optional from v9.5, it adds new instructions for atomic
memory operations with floating point values.  We have no immediate use
for it in kernel, provide a hwcap so userspace can discover it and allow
the ID register field to be exposed to KVM guests.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
Changes in v3:
- Rebase onto v6.17-rc1.
- Link to v2: https://lore.kernel.org/r/20250703-arm64-lsfe-v2-0-eced80999cb4@kernel.org

Changes in v2:
- Fix result of vi dropping in hwcap test.
- Link to v1: https://lore.kernel.org/r/20250627-arm64-lsfe-v1-0-68351c4bf741@kernel.org

---
Mark Brown (3):
      arm64/hwcap: Add hwcap for FEAT_LSFE
      KVM: arm64: Expose FEAT_LSFE to guests
      kselftest/arm64: Add lsfe to the hwcaps test

 Documentation/arch/arm64/elf_hwcaps.rst   |  4 ++++
 arch/arm64/include/asm/hwcap.h            |  1 +
 arch/arm64/include/uapi/asm/hwcap.h       |  1 +
 arch/arm64/kernel/cpufeature.c            |  2 ++
 arch/arm64/kernel/cpuinfo.c               |  1 +
 arch/arm64/kvm/sys_regs.c                 |  4 +++-
 tools/testing/selftests/arm64/abi/hwcap.c | 21 +++++++++++++++++++++
 7 files changed, 33 insertions(+), 1 deletion(-)
---
base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
change-id: 20250625-arm64-lsfe-0810cf98adc2

Best regards,
--  
Mark Brown <broonie@kernel.org>

[PATCH v3 1/3] arm64/hwcap: Add hwcap for FEAT_LSFE

[Mark Brown]

FEAT_LSFE (Large System Float Extension), providing atomic floating point
memory operations, is optional from v9.5. This feature adds no new
architectural stare and we have no immediate use for it in the kernel so
simply provide a hwcap for it to support discovery by userspace.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 Documentation/arch/arm64/elf_hwcaps.rst | 4 ++++
 arch/arm64/include/asm/hwcap.h          | 1 +
 arch/arm64/include/uapi/asm/hwcap.h     | 1 +
 arch/arm64/kernel/cpufeature.c          | 2 ++
 arch/arm64/kernel/cpuinfo.c             | 1 +
 5 files changed, 9 insertions(+)

diff --git a/Documentation/arch/arm64/elf_hwcaps.rst b/Documentation/arch/arm64/elf_hwcaps.rst
index f58ada4d6cb2..a15df4956849 100644
--- a/Documentation/arch/arm64/elf_hwcaps.rst
+++ b/Documentation/arch/arm64/elf_hwcaps.rst
@@ -441,6 +441,10 @@ HWCAP3_MTE_FAR
 HWCAP3_MTE_STORE_ONLY
     Functionality implied by ID_AA64PFR2_EL1.MTESTOREONLY == 0b0001.
 
+HWCAP3_LSFE
+    Functionality implied by ID_AA64ISAR3_EL1.LSFE == 0b0001
+
+
 4. Unused AT_HWCAP bits
 -----------------------
 
diff --git a/arch/arm64/include/asm/hwcap.h b/arch/arm64/include/asm/hwcap.h
index 13f94c8ddfc0..6d567265467c 100644
--- a/arch/arm64/include/asm/hwcap.h
+++ b/arch/arm64/include/asm/hwcap.h
@@ -178,6 +178,7 @@
 #define __khwcap3_feature(x)		(const_ilog2(HWCAP3_ ## x) + 128)
 #define KERNEL_HWCAP_MTE_FAR		__khwcap3_feature(MTE_FAR)
 #define KERNEL_HWCAP_MTE_STORE_ONLY	__khwcap3_feature(MTE_STORE_ONLY)
+#define KERNEL_HWCAP_LSFE		__khwcap3_feature(LSFE)
 
 /*
  * This yields a mask that user programs can use to figure out what
diff --git a/arch/arm64/include/uapi/asm/hwcap.h b/arch/arm64/include/uapi/asm/hwcap.h
index 72c78468b806..575564ecdb0b 100644
--- a/arch/arm64/include/uapi/asm/hwcap.h
+++ b/arch/arm64/include/uapi/asm/hwcap.h
@@ -145,5 +145,6 @@
  */
 #define HWCAP3_MTE_FAR		(1UL << 0)
 #define HWCAP3_MTE_STORE_ONLY		(1UL << 1)
+#define HWCAP3_LSFE		(1UL << 2)
 
 #endif /* _UAPI__ASM_HWCAP_H */
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 9ad065f15f1d..b1219f14459f 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -278,6 +278,7 @@ static const struct arm64_ftr_bits ftr_id_aa64isar2[] = {
 
 static const struct arm64_ftr_bits ftr_id_aa64isar3[] = {
 	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64ISAR3_EL1_FPRCVT_SHIFT, 4, 0),
+	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64ISAR3_EL1_LSFE_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64ISAR3_EL1_FAMINMAX_SHIFT, 4, 0),
 	ARM64_FTR_END,
 };
@@ -3252,6 +3253,7 @@ static const struct arm64_cpu_capabilities arm64_elf_hwcaps[] = {
 	HWCAP_CAP(ID_AA64ISAR1_EL1, I8MM, IMP, CAP_HWCAP, KERNEL_HWCAP_I8MM),
 	HWCAP_CAP(ID_AA64ISAR2_EL1, LUT, IMP, CAP_HWCAP, KERNEL_HWCAP_LUT),
 	HWCAP_CAP(ID_AA64ISAR3_EL1, FAMINMAX, IMP, CAP_HWCAP, KERNEL_HWCAP_FAMINMAX),
+	HWCAP_CAP(ID_AA64ISAR3_EL1, LSFE, IMP, CAP_HWCAP, KERNEL_HWCAP_LSFE),
 	HWCAP_CAP(ID_AA64MMFR2_EL1, AT, IMP, CAP_HWCAP, KERNEL_HWCAP_USCAT),
 #ifdef CONFIG_ARM64_SVE
 	HWCAP_CAP(ID_AA64PFR0_EL1, SVE, IMP, CAP_HWCAP, KERNEL_HWCAP_SVE),
diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
index ba834909a28b..c44e6d94f5de 100644
--- a/arch/arm64/kernel/cpuinfo.c
+++ b/arch/arm64/kernel/cpuinfo.c
@@ -162,6 +162,7 @@ static const char *const hwcap_str[] = {
 	[KERNEL_HWCAP_SME_SMOP4]	= "smesmop4",
 	[KERNEL_HWCAP_MTE_FAR]		= "mtefar",
 	[KERNEL_HWCAP_MTE_STORE_ONLY]	= "mtestoreonly",
+	[KERNEL_HWCAP_LSFE]		= "lsfe",
 };
 
 #ifdef CONFIG_COMPAT

-- 
2.39.5

[PATCH v3 2/3] KVM: arm64: Expose FEAT_LSFE to guests

[Mark Brown]

FEAT_LSFE (Large System Float Extension), providing atomic floating point
memory operations, is optional from v9.5. This feature adds no new
architectural state, expose the relevant ID register field to guests so
they can discover it.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/kvm/sys_regs.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 82ffb3b3b3cf..08e2cb94bff3 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1642,7 +1642,8 @@ static u64 __kvm_read_sanitised_id_reg(const struct kvm_vcpu *vcpu,
 			val &= ~ARM64_FEATURE_MASK(ID_AA64ISAR2_EL1_WFxT);
 		break;
 	case SYS_ID_AA64ISAR3_EL1:
-		val &= ID_AA64ISAR3_EL1_FPRCVT | ID_AA64ISAR3_EL1_FAMINMAX;
+		val &= ID_AA64ISAR3_EL1_FPRCVT | ID_AA64ISAR3_EL1_LSFE |
+			ID_AA64ISAR3_EL1_FAMINMAX;
 		break;
 	case SYS_ID_AA64MMFR2_EL1:
 		val &= ~ID_AA64MMFR2_EL1_CCIDX_MASK;
@@ -2991,6 +2992,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 					ID_AA64ISAR2_EL1_APA3 |
 					ID_AA64ISAR2_EL1_GPA3)),
 	ID_WRITABLE(ID_AA64ISAR3_EL1, (ID_AA64ISAR3_EL1_FPRCVT |
+				       ID_AA64ISAR3_EL1_LSFE |
 				       ID_AA64ISAR3_EL1_FAMINMAX)),
 	ID_UNALLOCATED(6,4),
 	ID_UNALLOCATED(6,5),

-- 
2.39.5

[PATCH v3 3/3] kselftest/arm64: Add lsfe to the hwcaps test

[Mark Brown]

This feature has no traps associated with it so the SIGILL is not reliable.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 tools/testing/selftests/arm64/abi/hwcap.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/tools/testing/selftests/arm64/abi/hwcap.c b/tools/testing/selftests/arm64/abi/hwcap.c
index 002ec38a8bbb..941890f69df6 100644
--- a/tools/testing/selftests/arm64/abi/hwcap.c
+++ b/tools/testing/selftests/arm64/abi/hwcap.c
@@ -17,6 +17,8 @@
 #include <asm/sigcontext.h>
 #include <asm/unistd.h>
 
+#include <linux/auxvec.h>
+
 #include "../../kselftest.h"
 
 #define TESTS_PER_HWCAP 3
@@ -169,6 +171,18 @@ static void lse128_sigill(void)
 		     : "cc", "memory");
 }
 
+static void lsfe_sigill(void)
+{
+	float __attribute__ ((aligned (16))) mem = 0;
+	register float *memp asm ("x0") = &mem;
+
+	/* LDFADD H0, H0, [X0] */
+	asm volatile(".inst 0x7c200000"
+		     : "+r" (memp)
+		     :
+		     : "cc", "memory");
+}
+
 static void lut_sigill(void)
 {
 	/* LUTI2 V0.16B, { V0.16B }, V[0] */
@@ -762,6 +776,13 @@ static const struct hwcap_data {
 		.cpuinfo = "lse128",
 		.sigill_fn = lse128_sigill,
 	},
+	{
+		.name = "LSFE",
+		.at_hwcap = AT_HWCAP3,
+		.hwcap_bit = HWCAP3_LSFE,
+		.cpuinfo = "lsfe",
+		.sigill_fn = lsfe_sigill,
+	},
 	{
 		.name = "LUT",
 		.at_hwcap = AT_HWCAP2,

-- 
2.39.5

Re: [PATCH v3 3/3] kselftest/arm64: Add lsfe to the hwcaps test

[Will Deacon]

On Mon, Aug 18, 2025 at 08:21:20PM +0100, Mark Brown wrote:
> This feature has no traps associated with it so the SIGILL is not reliable.
> 
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  tools/testing/selftests/arm64/abi/hwcap.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/tools/testing/selftests/arm64/abi/hwcap.c b/tools/testing/selftests/arm64/abi/hwcap.c
> index 002ec38a8bbb..941890f69df6 100644
> --- a/tools/testing/selftests/arm64/abi/hwcap.c
> +++ b/tools/testing/selftests/arm64/abi/hwcap.c
> @@ -17,6 +17,8 @@
>  #include <asm/sigcontext.h>
>  #include <asm/unistd.h>
>  
> +#include <linux/auxvec.h>
> +
>  #include "../../kselftest.h"
>  
>  #define TESTS_PER_HWCAP 3
> @@ -169,6 +171,18 @@ static void lse128_sigill(void)
>  		     : "cc", "memory");
>  }
>  
> +static void lsfe_sigill(void)
> +{
> +	float __attribute__ ((aligned (16))) mem = 0;
> +	register float *memp asm ("x0") = &mem;
> +
> +	/* LDFADD H0, H0, [X0] */
> +	asm volatile(".inst 0x7c200000"
> +		     : "+r" (memp)

Doesn't this corrupt H0 without the compiler knowing? It's probably
easier to use STFADD.

> +		     :
> +		     : "cc", "memory");

Why do you need the "cc" clobber?

Will

Re: [PATCH v3 3/3] kselftest/arm64: Add lsfe to the hwcaps test

[Mark Brown]

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On Tue, Sep 16, 2025 at 10:16:11PM +0100, Will Deacon wrote:
> On Mon, Aug 18, 2025 at 08:21:20PM +0100, Mark Brown wrote:

> > +	/* LDFADD H0, H0, [X0] */
> > +	asm volatile(".inst 0x7c200000"
> > +		     : "+r" (memp)

> Doesn't this corrupt H0 without the compiler knowing? It's probably
> easier to use STFADD.

Yeah, that's more correct and easier than specifying constraints.  In
practice it should be safe as the compiler is unlikely to use FP in the
instructions it generates and it's a caller saved register.  The program
is in general not careful with constraints.

> > +		     :
> > +		     : "cc", "memory");

> Why do you need the "cc" clobber?

It's overkill.

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Re: [PATCH v3 0/3] arm64: Support FEAT_LSFE (Large System Float Extension)

[Will Deacon]

On Mon, 18 Aug 2025 20:21:17 +0100, Mark Brown wrote:
> FEAT_LSFE is optional from v9.5, it adds new instructions for atomic
> memory operations with floating point values.  We have no immediate use
> for it in kernel, provide a hwcap so userspace can discover it and allow
> the ID register field to be exposed to KVM guests.
> 
> 

Applied first patch to arm64 (for-next/cpufeature), thanks!

[1/3] arm64/hwcap: Add hwcap for FEAT_LSFE
      https://git.kernel.org/arm64/c/220928e52cb0

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev