[PATCH] riscv: Add sysctl to control discard of vstate during syscall

* [PATCH] riscv: Add sysctl to control discard of vstate during syscall
@ 2025-07-19  3:39 Drew Fustini
  2025-07-21 12:13 ` Darius Rad
                   ` (2 more replies)
  0 siblings, 3 replies; 18+ messages in thread
From: Drew Fustini @ 2025-07-19  3:39 UTC (permalink / raw)
  To: Palmer Dabbelt, Björn Töpel, Alexandre Ghiti,
	Paul Walmsley, Samuel Holland, Drew Fustini, Andy Chiu,
	Conor Dooley, linux-riscv, linux-kernel
  Cc: Drew Fustini

From: Drew Fustini <dfustini@tenstorrent.com>

Clobbering the vector registers can significantly increase system call
latency for some implementations. To mitigate this performance impact, a
policy mechanism is provided to administrators, distro maintainers, and
developers to control vector state discard in the form of a sysctl knob:

/proc/sys/abi/riscv_v_vstate_discard

Valid values are:

0: Do not discard vector state during syscall
1: Discard vector state during syscall

The initial state is controlled by CONFIG_RISCV_ISA_V_VSTATE_DISCARD.

Fixes: 9657e9b7d253 ("riscv: Discard vector state on syscalls")
Signed-off-by: Drew Fustini <dfustini@tenstorrent.com>
---
 Documentation/arch/riscv/vector.rst | 15 +++++++++++++++
 arch/riscv/Kconfig                  | 10 ++++++++++
 arch/riscv/include/asm/vector.h     |  4 ++++
 arch/riscv/kernel/vector.c          | 16 +++++++++++++++-
 4 files changed, 44 insertions(+), 1 deletion(-)

I've tested the impact of riscv_v_vstate_discard() on the SiFive X280
cores [1] in the Tenstorrent Blackhole SoC [2]. The results from the
Blackhole P100 [3] card show that discarding the vector registers
increases null syscall latency by 25%.

The null syscall program [4] executes vsetvli and then calls getppid()
in a loop. The average duration of getppid() is 198 ns when registers
are clobbered in riscv_v_vstate_discard(). The average duration drops
to 149 ns when riscv_v_vstate_discard() skips clobbering the registers
as result of riscv_v_vstate_discard being set to 0.

$ sudo sysctl abi.riscv_v_vstate_discard=1
abi.riscv_v_vstate_discard = 1

$ ./null_syscall --vsetvli
vsetvli complete
 iterations: 1000000000
   duration: 198 seconds
avg latency: 198.73 ns

$ sudo sysctl abi.riscv_v_vstate_discard=0
abi.riscv_v_vstate_discard = 0

$ ./null_syscall --vsetvli
vsetvli complete
 iterations: 1000000000
   duration: 149 seconds
avg latency: 149.89 ns

I'm testing on the tt-blackhole-v6.16-rc1_vstate_discard [5] branch that
has 13 patches, including this one, on top of v6.16-rc1. Most are simple
yaml patches for dt bindings along with dts files and a bespoke network
driver. I don't think the other patches are relevant to this discussion.

This patch applies clean on its own to riscv/for-next and next-20250718.

[1] https://www.sifive.com/cores/intelligence-x200-series
[2] https://tenstorrent.com/en/hardware/blackhole
[3] https://github.com/tenstorrent/tt-bh-linux
[4] https://gist.github.com/tt-fustini/ab9b217756912ce75522b3cce11d0d58
[5] https://github.com/tenstorrent/linux/tree/tt-blackhole-v6.16-rc1_vstate_discard

diff --git a/Documentation/arch/riscv/vector.rst b/Documentation/arch/riscv/vector.rst
index 3987f5f76a9d..1edbce436015 100644
--- a/Documentation/arch/riscv/vector.rst
+++ b/Documentation/arch/riscv/vector.rst
@@ -137,4 +137,19 @@ processes in form of sysctl knob:
 As indicated by version 1.0 of the V extension [1], vector registers are
 clobbered by system calls.
 
+Clobbering the vector registers can significantly increase system call latency
+for some implementations. To mitigate the performance impact, a policy mechanism
+is provided to the administrators, distro maintainers, and developers to control
+the vstate discard in the form of a sysctl knob:
+
+* /proc/sys/abi/riscv_v_vstate_discard
+
+    Valid values are:
+
+    * 0: Do not discard vector state during syscall
+    * 1: Discard vector state during syscall
+
+    Reading this file returns the current discard 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
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 0aeee50da016..c0039f21d1f0 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -655,6 +655,16 @@ 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
+	  Say Y here if you want to enable Vector state discard on syscall.
+	  Otherwise, userspace has to enable it via the sysctl interface.
+
+	  If you don't know what to do here, say N.
+
 config RISCV_ISA_V_UCOPY_THRESHOLD
 	int "Threshold size for vectorized user copies"
 	depends on RISCV_ISA_V
diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
index 45c9b426fcc5..77991013216b 100644
--- a/arch/riscv/include/asm/vector.h
+++ b/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");
diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
index 184f780c932d..7a4c209ad337 100644
--- a/arch/riscv/kernel/vector.c
+++ b/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;
 }
 
-- 
2.34.1


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