The zawrs extension adds a pair of instructions that stall a core until
a memory location is written to. This patch uses one of them to
implement RISCV-specific versions of the rte_wait_until_equal_*
functions. This is potentially more energy efficient than the default
implementation that uses rte_pause/Zihintpause.
The technique works as follows:
* Create a reservation set containing the address we want to wait on
using an atomic load (lr.dw)
* Call wrs.nto - this blocks until the reservation set is invalidated by
someone else writing to that address
* Execution can also resume arbitrarily, so we still need to check
whether a change occurred and loop if not
Due to RISC-V atomics only supporting naturally aligned word (32 bit)
and double word (64 bit) loads, I've used pointer rounding and bit
shifting to implement waiting on 16-bit values.
This new functionality is controlled by a Meson flag that is disabled by
default.
Signed-off-by: Daniel Gregory <daniel.greg...@bytedance.com>
Suggested-by: Punit Agrawal <punit.agra...@bytedance.com>
---
Posting as an RFC to get early feedback and enable testing by others
with Zawrs-enabled hardware. Whilst I have been able to test it compiles
& passes tests using QEMU, I am waiting on some Zawrs-enabled hardware
to become available before I carry out performance tests.
Nonetheless, I would be glad to hear any feedback on the general
approach. Thanks, Daniel
config/riscv/meson.build | 5 ++
lib/eal/riscv/include/rte_pause.h | 139 ++++++++++++++++++++++++++++++
2 files changed, 144 insertions(+)
diff --git a/config/riscv/meson.build b/config/riscv/meson.build
index 07d7d9da23..4cfdc42ecb 100644
--- a/config/riscv/meson.build
+++ b/config/riscv/meson.build
@@ -26,6 +26,11 @@ flags_common = [
# read from /proc/device-tree/cpus/timebase-frequency. This property is
# guaranteed on Linux, as riscv time_init() requires it.
['RTE_RISCV_TIME_FREQ', 0],
+
+ # Enable use of RISC-V Wait-on-Reservation-Set extension (Zawrs)
+ # Mitigates looping when polling on memory locations
+ # Make sure to add '_zawrs' to your target's -march below
+ ['RTE_RISCV_ZAWRS', false]
]
## SoC-specific options.
diff --git a/lib/eal/riscv/include/rte_pause.h
b/lib/eal/riscv/include/rte_pause.h
index cb8e9ca52d..e7b43dffa3 100644
--- a/lib/eal/riscv/include/rte_pause.h
+++ b/lib/eal/riscv/include/rte_pause.h
@@ -11,6 +11,12 @@
extern "C" {
#endif
+#ifdef RTE_RISCV_ZAWRS
+#define RTE_WAIT_UNTIL_EQUAL_ARCH_DEFINED
+#endif
+
+#include <rte_debug.h>
+
#include "rte_atomic.h"
#include "generic/rte_pause.h"
@@ -24,6 +30,139 @@ static inline void rte_pause(void)
asm volatile(".int 0x0100000F" : : : "memory");
}
+#ifdef RTE_RISCV_ZAWRS
+
+/*
+ * Atomic load from an address, it returns either a sign-extended word or
+ * doubleword and creates a 'reservation set' containing the read memory
+ * location. When someone else writes to the reservation set, it is
invalidated,
+ * causing any stalled WRS instructions to resume.
+ *
+ * Address needs to be naturally aligned.
+ */
+#define __RTE_RISCV_LR_32(src, dst, memorder) do { \
+ if ((memorder) == rte_memory_order_relaxed) { \
+ asm volatile("lr.w %0, (%1)" \
+ : "=r" (dst) \
+ : "r" (src) \
+ : "memory"); \
+ } else { \
+ asm volatile("lr.w.aq %0, (%1)" \
+ : "=r" (dst) \
+ : "r" (src) \
+ : "memory"); \
+ } } while (0)
+#define __RTE_RISCV_LR_64(src, dst, memorder) do { \
+ if ((memorder) == rte_memory_order_relaxed) { \
+ asm volatile("lr.d %0, (%1)" \
+ : "=r" (dst) \
+ : "r" (src) \
+ : "memory"); \
+ } else { \
+ asm volatile("lr.d.aq %0, (%1)" \
+ : "=r" (dst) \
+ : "r" (src) \
+ : "memory"); \
+ } } while (0)
+
+/*
+ * There's not a RISC-V atomic load primitive for halfwords, so cast up to a
+ * _naturally aligned_ word and extract the halfword we want
+ */
+#define __RTE_RISCV_LR_16(src, dst, memorder) do { \
+ uint32_t word; \
+ __RTE_RISCV_LR_32(((uintptr_t)(src) & (~3)), word, (memorder)); \
+ if ((size_t)(src) & 3) \
+ (dst) = (typeof(dst))(word >> 16); \
+ else \
+ (dst) = (typeof(dst))(word & 0xFFFF); \
+} while (0)
+
+#define __RTE_RISCV_LR(src, dst, memorder, size) { \
+ RTE_BUILD_BUG_ON(size != 16 && size != 32 && size != 64); \
+ if (size == 16) \
+ __RTE_RISCV_LR_16(src, dst, memorder); \
+ else if (size == 32) \
+ __RTE_RISCV_LR_32(src, dst, memorder); \
+ else if (size == 64) \
+ __RTE_RISCV_LR_64(src, dst, memorder); \
+}
+
+/*
+ * Wait-on-Reservation-Set extension instruction, it stalls execution until the
+ * reservation set is invalidated or an interrupt is observed.
+ * A loop is likely still needed as it may stop stalling arbitrarily.
+ */
+#define __RTE_RISCV_WRS_NTO() { asm volatile("wrs.nto" : : : "memory"); }
+
+static __rte_always_inline void
+rte_wait_until_equal_16(volatile uint16_t *addr, uint16_t expected,
+ int memorder)
+{
+ uint16_t value;
+
+ RTE_ASSERT(memorder == rte_memory_order_acquire ||
+ memorder == rte_memory_order_relaxed);
+ RTE_ASSERT(((size_t)addr & 1) == 0);
+
+ __RTE_RISCV_LR_16(addr, value, memorder);
+ while (value != expected) {
+ __RTE_RISCV_WRS_NTO();
+ __RTE_RISCV_LR_16(addr, value, memorder);
+ }
+}
+
+static __rte_always_inline void
+rte_wait_until_equal_32(volatile uint32_t *addr, uint32_t expected,
+ int memorder)
+{
+ uint32_t value;
+
+ RTE_ASSERT(memorder == rte_memory_order_acquire ||
+ memorder == rte_memory_order_relaxed);
+ RTE_ASSERT(((size_t)addr & 3) == 0);
+
+ __RTE_RISCV_LR_32(addr, value, memorder);
+ while (value != expected) {
+ __RTE_RISCV_WRS_NTO();
+ __RTE_RISCV_LR_32(addr, value, memorder);
+ }
+}
+
+static __rte_always_inline void
+rte_wait_until_equal_64(volatile uint64_t *addr, uint64_t expected,
+ int memorder)
+{
+ uint64_t value;
+
+ RTE_ASSERT(memorder == rte_memory_order_acquire ||
+ memorder == rte_memory_order_relaxed);
+ RTE_ASSERT(((size_t)addr & 7) == 0);
+
+ __RTE_RISCV_LR_64(addr, value, memorder);
+ while (value != expected) {
+ __RTE_RISCV_WRS_NTO();
+ __RTE_RISCV_LR_64(addr, value, memorder);
+ }
+}
+
+#define RTE_WAIT_UNTIL_MASKED(addr, mask, cond, expected, memorder) do { \
+ RTE_BUILD_BUG_ON(!__builtin_constant_p(memorder)); \
+ RTE_BUILD_BUG_ON(memorder != rte_memory_order_acquire && \
+ memorder != rte_memory_order_relaxed); \
+ RTE_ASSERT(((size_t)(addr) & (sizeof(*(addr)) - 1)) != 0); \
+ const uint32_t size = sizeof(*(addr)) << 3; \
+ typeof(*(addr)) expected_value = (expected); \
+ typeof(*(addr)) value; \
+ __RTE_RISCV_LR((addr), value, memorder, size); \
+ while (!((value & (mask)) cond expected_value)) { \
+ __RTE_RISCV_WRS_NTO(); \
+ __RTE_RISCV_LR((addr), value, memorder, size); \
+ } \
+} while (0)
+
+#endif /* RTE_RISCV_ZAWRS */
+
#ifdef __cplusplus
}
#endif
--
2.39.2
