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Forwarded: https://github.com/MariaDB/server/pull/2980 (merged in Feb 2024)
Origin: https://patch-diff.githubusercontent.com/raw/MariaDB/server/pull/2980.patch
From: Aurelien Jarno <aurelien@aurel32.net>
Date: Thu, 4 Jan 2024 11:30:34 +0100
Subject: [PATCH] RISC-V: use RDTIME instead of RDCYCLE
Starting with Linux 6.6 [1], RDCYCLE is a privileged instruction on
RISC-V and can't be used directly from userland. There is a sysctl
option to change that as a transition period, but it will eventually
disappear.
Use RDTIME instead, which while less accurate has the advantage of being
synchronized between CPU (and thus monotonic) and of constant frequency.
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cc4c07c89aada16229084eeb93895c95b7eabaa3
---
include/my_rdtsc.h | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
--- a/include/my_rdtsc.h
+++ b/include/my_rdtsc.h
@@ -111,7 +111,7 @@ C_MODE_START
On AARCH64, we use the generic timer base register. We override clang
implementation for aarch64 as it access a PMU register which is not
guaranteed to be active.
- On RISC-V, we use the rdcycle instruction to read from mcycle register.
+ On RISC-V, we use the rdtime instruction to read from mtime register.
Sadly, we have nothing for the Digital Alpha, MIPS, Motorola m68k,
HP PA-RISC or other non-mainstream (or obsolete) processors.
@@ -211,15 +211,15 @@ static inline ulonglong my_timer_cycles(
}
#elif defined(__riscv)
#define MY_TIMER_ROUTINE_CYCLES MY_TIMER_ROUTINE_RISCV
- /* Use RDCYCLE (and RDCYCLEH on riscv32) */
+ /* Use RDTIME (and RDTIMEH on riscv32) */
{
# if __riscv_xlen == 32
ulong result_lo, result_hi0, result_hi1;
/* Implemented in assembly because Clang insisted on branching. */
__asm __volatile__(
- "rdcycleh %0\n"
- "rdcycle %1\n"
- "rdcycleh %2\n"
+ "rdtimeh %0\n"
+ "rdtime %1\n"
+ "rdtimeh %2\n"
"sub %0, %0, %2\n"
"seqz %0, %0\n"
"sub %0, zero, %0\n"
@@ -228,7 +228,7 @@ static inline ulonglong my_timer_cycles(
return (static_cast<ulonglong>(result_hi1) << 32) | result_lo;
# else
ulonglong result;
- __asm __volatile__("rdcycle %0" : "=r"(result));
+ __asm __volatile__("rdtime %0" : "=r"(result));
return result;
}
# endif
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