[PATCH v2 2/2] arm64: smccc: Support SMCCC v1.3 SVE register saving hint

[Mark Brown <broonie@kernel.org>]

SMCCC v1.2 requires that all SVE state be preserved over SMC calls which
introduces substantial overhead in the common case where there is no SVE
state in the registers. To avoid this SMCCC v1.3 introduces a flag which
allows the caller to say that there is no state that needs to be preserved
in the registers. Make use of this flag, setting it if the SMCCC version
indicates support for it and the TIF_ flags indicate that there is no live
SVE state in the registers, this avoids placing any constraints on when
SMCCC calls can be done or triggering extra saving and reloading of SVE
register state in the kernel.

This would be straightforward enough except for the rather entertaining
inline assembly we use to do SMCCC v1.1 calls to allow us to take advantage
of the limited number of registers it clobbers. Deal with this by having a
slightly non-standard function which we call immediately before issuing the
SMCCC call to make our checks. This causes an extra function call on any
system built with SVE support, and extra checks when SVE is detected at
runtime, but these costs are expected to be reasonable in the context of
doing a SMCCC call in the first place.

Since we can't check the TIF_ flags for current and would need to map
the helper function separately when in the nVHE hypervisor we don't
attempt to do the optimisation for it, this could be done as a followup.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/kernel/smccc-call.S | 38 ++++++++++++++++++++++++++++++++++
 drivers/firmware/smccc/smccc.c |  4 ++++
 include/linux/arm-smccc.h      | 23 ++++++++++++++++++--
 3 files changed, 63 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/smccc-call.S b/arch/arm64/kernel/smccc-call.S
index d62447964ed9..2713ea37b536 100644
--- a/arch/arm64/kernel/smccc-call.S
+++ b/arch/arm64/kernel/smccc-call.S
@@ -7,8 +7,46 @@
 
 #include <asm/asm-offsets.h>
 #include <asm/assembler.h>
+#include <asm/thread_info.h>
+
+/*
+ * If we have SMCCC v1.3 and (as is likely) no SVE state in
+ * the registers then set the SMCCC hint bit to say there's no
+ * need to preserve it.  Do this by directly adjusting the SMCCC
+ * function value which is already stored in x0 ready to be called.
+ *
+ * Since we need scratch registers but wish to avoid having to handle
+ * the stack we expect the caller to preserve x15 and x16 if needed,
+ * the only callers are expected to be the call below and the inline
+ * asm in linux/arm-smccc.h for SMCCC 1.1 and later calls.
+ */
+SYM_CODE_START(__smccc_sve_check)
+	BTI_C
+
+alternative_if ARM64_SVE
+
+	ldr_l	x15, smccc_has_sve_hint
+	cbz	x15, 2f
+
+	get_current_task x15
+	ldr	x15, [x15, #TSK_TI_FLAGS]
+	and	x16, x15, #_TIF_FOREIGN_FPSTATE	// Any live FP state?
+	cbnz	x16, 1f
+	mov	x16, #_TIF_SVE			// Does that state include SVE?
+	and	x16, x15, x16
+	cbnz	x16, 2f
+
+1:	orr	x0, x0, ARM_SMCCC_1_3_SVE_HINT
+alternative_else_nop_endif
+
+2:	ret
+SYM_CODE_END(__smccc_sve_check)
+EXPORT_SYMBOL(__smccc_sve_check)
 
 	.macro SMCCC instr
+alternative_if ARM64_SVE
+	bl	__smccc_sve_check
+alternative_else_nop_endif
 	\instr	#0
 	ldr	x4, [sp]
 	stp	x0, x1, [x4, #ARM_SMCCC_RES_X0_OFFS]
diff --git a/drivers/firmware/smccc/smccc.c b/drivers/firmware/smccc/smccc.c
index 028f81d702cc..9f937b125ab0 100644
--- a/drivers/firmware/smccc/smccc.c
+++ b/drivers/firmware/smccc/smccc.c
@@ -15,6 +15,7 @@ static u32 smccc_version = ARM_SMCCC_VERSION_1_0;
 static enum arm_smccc_conduit smccc_conduit = SMCCC_CONDUIT_NONE;
 
 bool __ro_after_init smccc_trng_available = false;
+u64 __ro_after_init smccc_has_sve_hint = false;
 
 void __init arm_smccc_version_init(u32 version, enum arm_smccc_conduit conduit)
 {
@@ -22,6 +23,9 @@ void __init arm_smccc_version_init(u32 version, enum arm_smccc_conduit conduit)
 	smccc_conduit = conduit;
 
 	smccc_trng_available = smccc_probe_trng();
+	if (IS_ENABLED(CONFIG_ARM64_SVE) &&
+	    smccc_version >= ARM_SMCCC_VERSION_1_3)
+		smccc_has_sve_hint = true;
 }
 
 enum arm_smccc_conduit arm_smccc_1_1_get_conduit(void)
diff --git a/include/linux/arm-smccc.h b/include/linux/arm-smccc.h
index 6861489a1890..9e974a9f8993 100644
--- a/include/linux/arm-smccc.h
+++ b/include/linux/arm-smccc.h
@@ -63,6 +63,9 @@
 #define ARM_SMCCC_VERSION_1_0		0x10000
 #define ARM_SMCCC_VERSION_1_1		0x10001
 #define ARM_SMCCC_VERSION_1_2		0x10002
+#define ARM_SMCCC_VERSION_1_3		0x10003
+
+#define ARM_SMCCC_1_3_SVE_HINT		0x10000
 
 #define ARM_SMCCC_VERSION_FUNC_ID					\
 	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,				\
@@ -216,6 +219,8 @@ u32 arm_smccc_get_version(void);
 
 void __init arm_smccc_version_init(u32 version, enum arm_smccc_conduit conduit);
 
+extern u64 smccc_has_sve_hint;
+
 /**
  * struct arm_smccc_res - Result from SMC/HVC call
  * @a0-a3 result values from registers 0 to 3
@@ -297,6 +302,19 @@ asmlinkage void __arm_smccc_hvc(unsigned long a0, unsigned long a1,
 
 #endif
 
+/* nVHE hypervisor doesn't have a current thread so needs separate checks */
+#if defined(CONFIG_ARM64_SVE) && !defined(__KVM_NVHE_HYPERVISOR__)
+
+#define SMCCC_SVE_CHECK "bl __smccc_sve_check \n"
+#define smccc_sve_clobbers "x15", "x16", "lr",
+
+#else
+
+#define SMCCC_SVE_CHECK
+#define smccc_sve_clobbers
+
+#endif
+
 #define ___count_args(_0, _1, _2, _3, _4, _5, _6, _7, _8, x, ...) x
 
 #define __count_args(...)						\
@@ -364,7 +382,7 @@ asmlinkage void __arm_smccc_hvc(unsigned long a0, unsigned long a1,
 
 #define ___constraints(count)						\
 	: __constraint_read_ ## count					\
-	: "memory"
+	: smccc_sve_clobbers "memory"
 #define __constraints(count)	___constraints(count)
 
 /*
@@ -379,7 +397,8 @@ asmlinkage void __arm_smccc_hvc(unsigned long a0, unsigned long a1,
 		register unsigned long r2 asm("r2");			\
 		register unsigned long r3 asm("r3"); 			\
 		__declare_args(__count_args(__VA_ARGS__), __VA_ARGS__);	\
-		asm volatile(inst "\n" :				\
+		asm volatile(SMCCC_SVE_CHECK				\
+			     inst "\n" :				\
 			     "=r" (r0), "=r" (r1), "=r" (r2), "=r" (r3)	\
 			     __constraints(__count_args(__VA_ARGS__)));	\
 		if (___res)						\
-- 
2.20.1


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