[PW_SID:993002] [v3] riscv: Add sysctl to control discard of vstate on syscall entry

Documentation/arch/riscv/vector.rst

@@ -134,7 +134,30 @@ processes in form of sysctl knob:
 3.  Vector Register State Across System Calls
 ---------------------------------------------
 
-As indicated by version 1.0 of the V extension [1], vector registers are
-clobbered by system calls.
+Linux adopts the syscall ABI proposed by version 1.0 of the V extension [1],
+where vector registers are clobbered by system calls. Specifically:
+
+    Executing a system call causes all caller-saved vector registers
+    (v0-v31, vl, vtype) and vstart to become unspecified.
+
+Linux clobbers the vector registers (e.g. discards vector state) on the syscall
+entry path. This is done to identify userspace programs that mistakenly expect
+vector registers to be preserved across syscalls. This can be helpful for
+debugging and testing. However, clobbering vector state can negatively impact
+performance on some RISC-V implementations, and is not strictly necessary.
+
+To mitigate this performance impact, a sysctl knob is provided that controls
+whether vector state is always clobbered on syscall entry:
+
+* /proc/sys/abi/riscv_v_vstate_discard
+
+    Valid values are:
+
+    * 0: Vector state is not always clobbered in all syscalls
+    * 1: Mandatory clobbering of vector state in all syscalls
+
+    Reading this file returns the current discard behavior. Write to '0' or '1'
+    to file to change the current behavior. The initial state is controlled by
+    CONFIG_RISCV_ISA_V_VSTATE_DISCARD.
 
 1: https://github.com/riscv/riscv-v-spec/blob/master/calling-convention.adoc

arch/riscv/Kconfig

@@ -654,6 +654,26 @@ config RISCV_ISA_V_DEFAULT_ENABLE
 
 	  If you don't know what to do here, say Y.
 
+config RISCV_ISA_V_VSTATE_DISCARD
+	bool "Enable Vector state discard by default"
+	depends on RISCV_ISA_V
+	default n
+	help
+	  Discarding vector state (also known as clobbering) on syscall entry
+	  can help identify userspace programs that are mistakenly relying on
+	  vector registers being preserved across syscalls. This can be useful
+	  for debugging and testing. However, this behavior can negatively
+	  impact performance on some RISC-V implementations and is not strictly
+	  necessary.
+
+	  Select Y here if you want mandatory clobbering of vector state even
+	  though it can increase the duration of syscalls on some RISC-V cores.
+	  If you don't know what to do, then select N.
+
+	  This choice sets the initial value of the abi.riscv_v_vstate_discard
+	  sysctl. Regardless of whether you choose Y or N, the sysctl can still
+	  be changed by the user while the system is running.
+
 config RISCV_ISA_V_UCOPY_THRESHOLD
 	int "Threshold size for vectorized user copies"
 	depends on RISCV_ISA_V

arch/riscv/include/asm/vector.h

@@ -40,6 +40,7 @@
 	_res;								\
 })
 
+extern bool riscv_v_vstate_discard_ctl;
 extern unsigned long riscv_v_vsize;
 int riscv_v_setup_vsize(void);
 bool insn_is_vector(u32 insn_buf);
@@ -270,6 +271,9 @@ static inline void __riscv_v_vstate_discard(void)
 {
 	unsigned long vl, vtype_inval = 1UL << (BITS_PER_LONG - 1);
 
+	if (READ_ONCE(riscv_v_vstate_discard_ctl) == 0)
+		return;
+
 	riscv_v_enable();
 	if (has_xtheadvector())
 		asm volatile (THEAD_VSETVLI_T4X0E8M8D1 : : : "t4");

arch/riscv/kernel/vector.c

@@ -26,6 +26,7 @@ static struct kmem_cache *riscv_v_user_cachep;
 static struct kmem_cache *riscv_v_kernel_cachep;
 #endif
 
+bool riscv_v_vstate_discard_ctl = IS_ENABLED(CONFIG_RISCV_ISA_V_VSTATE_DISCARD);
 unsigned long riscv_v_vsize __read_mostly;
 EXPORT_SYMBOL_GPL(riscv_v_vsize);
 
@@ -307,11 +308,24 @@ static const struct ctl_table riscv_v_default_vstate_table[] = {
 	},
 };
 
+static const struct ctl_table riscv_v_vstate_discard_table[] = {
+	{
+		.procname       = "riscv_v_vstate_discard",
+		.data           = &riscv_v_vstate_discard_ctl,
+		.maxlen         = sizeof(riscv_v_vstate_discard_ctl),
+		.mode           = 0644,
+		.proc_handler   = proc_dobool,
+	},
+};
+
 static int __init riscv_v_sysctl_init(void)
 {
-	if (has_vector() || has_xtheadvector())
+	if (has_vector() || has_xtheadvector()) {
 		if (!register_sysctl("abi", riscv_v_default_vstate_table))
 			return -EINVAL;
+		if (!register_sysctl("abi", riscv_v_vstate_discard_table))
+			return -EINVAL;
+	}
 	return 0;
 }