arm64/sve: Fix missing SVE/FPSIMD endianness conversions

The in-memory representation of SVE and FPSIMD registers is
different: the FPSIMD V-registers are stored as single 128-bit
host-endian values, whereas SVE registers are stored in an
endianness-invariant byte order.

This means that the two representations differ when running on a
big-endian host.  But we blindly copy data from one representation
to another when converting between the two, resulting in the
register contents being unintentionally byteswapped in certain
situations.  Currently this can be triggered by the first SVE
instruction after a syscall, for example (though the potential
trigger points may vary in future).

So, fix the conversion functions fpsimd_to_sve(), sve_to_fpsimd()
and sve_sync_from_fpsimd_zeropad() to swab where appropriate.

There is no common swahl128() or swab128() that we could use here.
Maybe it would be worth making this generic, but for now add a
simple local hack.

Since the byte order differences are exposed in ABI, also clarify
the documentation.

Cc: Alex Bennée <alex.bennee@linaro.org>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Alan Hayward <alan.hayward@arm.com>
Cc: Julien Grall <julien.grall@arm.com>
Fixes: bc0ee47603 ("arm64/sve: Core task context handling")
Fixes: 8cd969d28f ("arm64/sve: Signal handling support")
Fixes: 43d4da2c45 ("arm64/sve: ptrace and ELF coredump support")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
[will: Fix typos in comments and docs spotted by Julien]
Signed-off-by: Will Deacon <will.deacon@arm.com>

Documentation/arm64/sve.txt

@@ -56,6 +56,18 @@ model features for SVE is included in Appendix A.
   is to connect to a target process first and then attempt a
   ptrace(PTRACE_GETREGSET, pid, NT_ARM_SVE, &iov).
 
+* Whenever SVE scalable register values (Zn, Pn, FFR) are exchanged in memory
+  between userspace and the kernel, the register value is encoded in memory in
+  an endianness-invariant layout, with bits [(8 * i + 7) : (8 * i)] encoded at
+  byte offset i from the start of the memory representation.  This affects for
+  example the signal frame (struct sve_context) and ptrace interface
+  (struct user_sve_header) and associated data.
+
+  Beware that on big-endian systems this results in a different byte order than
+  for the FPSIMD V-registers, which are stored as single host-endian 128-bit
+  values, with bits [(127 - 8 * i) : (120 - 8 * i)] of the register encoded at
+  byte offset i.  (struct fpsimd_context, struct user_fpsimd_state).
+
 
 2.  Vector length terminology
 -----------------------------
@@ -124,6 +136,10 @@ the SVE instruction set architecture.
   size and layout.  Macros SVE_SIG_* are defined [1] to facilitate access to
   the members.
 
+* Each scalable register (Zn, Pn, FFR) is stored in an endianness-invariant
+  layout, with bits [(8 * i + 7) : (8 * i)] stored at byte offset i from the
+  start of the register's representation in memory.
+
 * If the SVE context is too big to fit in sigcontext.__reserved[], then extra
   space is allocated on the stack, an extra_context record is written in
   __reserved[] referencing this space.  sve_context is then written in the

arch/arm64/include/uapi/asm/kvm.h

@@ -260,6 +260,13 @@ struct kvm_vcpu_events {
 	 KVM_REG_SIZE_U256 |						\
 	 ((i) & (KVM_ARM64_SVE_MAX_SLICES - 1)))
 
+/*
+ * Register values for KVM_REG_ARM64_SVE_ZREG(), KVM_REG_ARM64_SVE_PREG() and
+ * KVM_REG_ARM64_SVE_FFR() are represented in memory in an endianness-
+ * invariant layout which differs from the layout used for the FPSIMD
+ * V-registers on big-endian systems: see sigcontext.h for more explanation.
+ */
+
 #define KVM_ARM64_SVE_VQ_MIN __SVE_VQ_MIN
 #define KVM_ARM64_SVE_VQ_MAX __SVE_VQ_MAX
 

arch/arm64/include/uapi/asm/ptrace.h

@@ -176,6 +176,10 @@ struct user_sve_header {
  *	FPCR	uint32_t			FPCR
  *
  * Additional data might be appended in the future.
+ *
+ * The Z-, P- and FFR registers are represented in memory in an endianness-
+ * invariant layout which differs from the layout used for the FPSIMD
+ * V-registers on big-endian systems: see sigcontext.h for more explanation.
  */
 
 #define SVE_PT_SVE_ZREG_SIZE(vq)	__SVE_ZREG_SIZE(vq)

arch/arm64/include/uapi/asm/sigcontext.h

@@ -77,6 +77,15 @@ struct fpsimd_context {
 	__uint128_t vregs[32];
 };
 
+/*
+ * Note: similarly to all other integer fields, each V-register is stored in an
+ * endianness-dependent format, with the byte at offset i from the start of the
+ * in-memory representation of the register value containing
+ *
+ *    bits [(7 + 8 * i) : (8 * i)] of the register on little-endian hosts; or
+ *    bits [(127 - 8 * i) : (120 - 8 * i)] on big-endian hosts.
+ */
+
 /* ESR_EL1 context */
 #define ESR_MAGIC	0x45535201
 
@@ -204,6 +213,11 @@ struct sve_context {
  *	FFR	uint16_t[vq]			first-fault status register
  *
  * Additional data might be appended in the future.
+ *
+ * Unlike vregs[] in fpsimd_context, each SVE scalable register (Z-, P- or FFR)
+ * is encoded in memory in an endianness-invariant format, with the byte at
+ * offset i from the start of the in-memory representation containing bits
+ * [(7 + 8 * i) : (8 * i)] of the register value.
  */
 
 #define SVE_SIG_ZREG_SIZE(vq)	__SVE_ZREG_SIZE(vq)

arch/arm64/kernel/fpsimd.c

@@ -39,6 +39,7 @@
 #include <linux/slab.h>
 #include <linux/stddef.h>
 #include <linux/sysctl.h>
+#include <linux/swab.h>
 
 #include <asm/esr.h>
 #include <asm/fpsimd.h>
@@ -352,6 +353,23 @@ static int __init sve_sysctl_init(void) { return 0; }
 #define ZREG(sve_state, vq, n) ((char *)(sve_state) +		\
 	(SVE_SIG_ZREG_OFFSET(vq, n) - SVE_SIG_REGS_OFFSET))
 
+#ifdef CONFIG_CPU_BIG_ENDIAN
+static __uint128_t arm64_cpu_to_le128(__uint128_t x)
+{
+	u64 a = swab64(x);
+	u64 b = swab64(x >> 64);
+
+	return ((__uint128_t)a << 64) | b;
+}
+#else
+static __uint128_t arm64_cpu_to_le128(__uint128_t x)
+{
+	return x;
+}
+#endif
+
+#define arm64_le128_to_cpu(x) arm64_cpu_to_le128(x)
+
 /*
  * Transfer the FPSIMD state in task->thread.uw.fpsimd_state to
  * task->thread.sve_state.
@@ -369,14 +387,16 @@ static void fpsimd_to_sve(struct task_struct *task)
 	void *sst = task->thread.sve_state;
 	struct user_fpsimd_state const *fst = &task->thread.uw.fpsimd_state;
 	unsigned int i;
+	__uint128_t *p;
 
 	if (!system_supports_sve())
 		return;
 
 	vq = sve_vq_from_vl(task->thread.sve_vl);
-	for (i = 0; i < 32; ++i)
-		memcpy(ZREG(sst, vq, i), &fst->vregs[i],
-		       sizeof(fst->vregs[i]));
+	for (i = 0; i < 32; ++i) {
+		p = (__uint128_t *)ZREG(sst, vq, i);
+		*p = arm64_cpu_to_le128(fst->vregs[i]);
+	}
 }
 
 /*
@@ -395,14 +415,16 @@ static void sve_to_fpsimd(struct task_struct *task)
 	void const *sst = task->thread.sve_state;
 	struct user_fpsimd_state *fst = &task->thread.uw.fpsimd_state;
 	unsigned int i;
+	__uint128_t const *p;
 
 	if (!system_supports_sve())
 		return;
 
 	vq = sve_vq_from_vl(task->thread.sve_vl);
-	for (i = 0; i < 32; ++i)
-		memcpy(&fst->vregs[i], ZREG(sst, vq, i),
-		       sizeof(fst->vregs[i]));
+	for (i = 0; i < 32; ++i) {
+		p = (__uint128_t const *)ZREG(sst, vq, i);
+		fst->vregs[i] = arm64_le128_to_cpu(*p);
+	}
 }
 
 #ifdef CONFIG_ARM64_SVE
@@ -491,6 +513,7 @@ void sve_sync_from_fpsimd_zeropad(struct task_struct *task)
 	void *sst = task->thread.sve_state;
 	struct user_fpsimd_state const *fst = &task->thread.uw.fpsimd_state;
 	unsigned int i;
+	__uint128_t *p;
 
 	if (!test_tsk_thread_flag(task, TIF_SVE))
 		return;
@@ -499,9 +522,10 @@ void sve_sync_from_fpsimd_zeropad(struct task_struct *task)
 
 	memset(sst, 0, SVE_SIG_REGS_SIZE(vq));
 
-	for (i = 0; i < 32; ++i)
-		memcpy(ZREG(sst, vq, i), &fst->vregs[i],
-		       sizeof(fst->vregs[i]));
+	for (i = 0; i < 32; ++i) {
+		p = (__uint128_t *)ZREG(sst, vq, i);
+		*p = arm64_cpu_to_le128(fst->vregs[i]);
+	}
 }
 
 int sve_set_vector_length(struct task_struct *task,