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FFmpeg/libavutil/riscv/timer.h

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lavu/riscv: AV_READ_TIME cycle counter This uses the architected RISC-V 64-bit cycle counter from the RISC-V unprivileged instruction set. In 64-bit and 128-bit, this is a straightforward CSR read. In 32-bit mode, the 64-bit value is exposed as two CSRs, which cannot be read atomically, so a loop is necessary to detect and fix up the race condition where the bottom half wraps exactly between the two reads.
2 years ago
/*
* This file is part of FFmpeg.
*
* FFmpeg is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* FFmpeg is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with FFmpeg; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
#ifndef AVUTIL_RISCV_TIMER_H
#define AVUTIL_RISCV_TIMER_H
#include "config.h"
#if HAVE_INLINE_ASM
#include <stdint.h>
lavu/timer: use time for AV_READ_TIME on RISC-V So far, AV_READ_TIME would return the cycle counter. This posed two problems: 1) On recent systems, it would just raise an illegal instruction exception. Indeed RDCYCLE is blocked in user space to ward off some side channel attacks. In particular, this would cause the random number generator to crash. 2) It does not match the x86 behaviour and the apparent original intent of AV_READ_TIME in the functional code base (outside test cases). So this replaces the cycle counter with the time counter. The unit is a platform-dependent constant fraction of time, and the value should be stable across harts (RISC-V lingo for physical CPU thread).
1 year ago
static inline uint64_t ff_read_time(void)
lavu/riscv: AV_READ_TIME cycle counter This uses the architected RISC-V 64-bit cycle counter from the RISC-V unprivileged instruction set. In 64-bit and 128-bit, this is a straightforward CSR read. In 32-bit mode, the 64-bit value is exposed as two CSRs, which cannot be read atomically, so a loop is necessary to detect and fix up the race condition where the bottom half wraps exactly between the two reads.
2 years ago
{
#if (__riscv_xlen >= 64)
uintptr_t cycles;
Revert "lavu/timer: remove gratuitous volatile" It does not make much sense to me, but GCC somehow optimises the inline assembler even though the output is very obviously used and having observable side effects. This reverts commit 09731fbfc3a914ec4f6ffad60aa9062db6a8f6aa.
1 year ago
__asm__ volatile ("rdtime %0" : "=r" (cycles));
lavu/riscv: AV_READ_TIME cycle counter This uses the architected RISC-V 64-bit cycle counter from the RISC-V unprivileged instruction set. In 64-bit and 128-bit, this is a straightforward CSR read. In 32-bit mode, the 64-bit value is exposed as two CSRs, which cannot be read atomically, so a loop is necessary to detect and fix up the race condition where the bottom half wraps exactly between the two reads.
2 years ago
#else
uint64_t cycles;
uint32_t hi, lo, check;
Revert "lavu/timer: remove gratuitous volatile" It does not make much sense to me, but GCC somehow optimises the inline assembler even though the output is very obviously used and having observable side effects. This reverts commit 09731fbfc3a914ec4f6ffad60aa9062db6a8f6aa.
1 year ago
__asm__ volatile (
lavu/timer: use time for AV_READ_TIME on RISC-V So far, AV_READ_TIME would return the cycle counter. This posed two problems: 1) On recent systems, it would just raise an illegal instruction exception. Indeed RDCYCLE is blocked in user space to ward off some side channel attacks. In particular, this would cause the random number generator to crash. 2) It does not match the x86 behaviour and the apparent original intent of AV_READ_TIME in the functional code base (outside test cases). So this replaces the cycle counter with the time counter. The unit is a platform-dependent constant fraction of time, and the value should be stable across harts (RISC-V lingo for physical CPU thread).
1 year ago
"1: rdtimeh %0\n"
" rdtime %1\n"
" rdtimeh %2\n"
lavu/riscv: AV_READ_TIME cycle counter This uses the architected RISC-V 64-bit cycle counter from the RISC-V unprivileged instruction set. In 64-bit and 128-bit, this is a straightforward CSR read. In 32-bit mode, the 64-bit value is exposed as two CSRs, which cannot be read atomically, so a loop is necessary to detect and fix up the race condition where the bottom half wraps exactly between the two reads.
2 years ago
" bne %0, %2, 1b\n" : "=r" (hi), "=r" (lo), "=r" (check));
cycles = (((uint64_t)hi) << 32) | lo;
#endif
return cycles;
}
lavu/timer: use time for AV_READ_TIME on RISC-V So far, AV_READ_TIME would return the cycle counter. This posed two problems: 1) On recent systems, it would just raise an illegal instruction exception. Indeed RDCYCLE is blocked in user space to ward off some side channel attacks. In particular, this would cause the random number generator to crash. 2) It does not match the x86 behaviour and the apparent original intent of AV_READ_TIME in the functional code base (outside test cases). So this replaces the cycle counter with the time counter. The unit is a platform-dependent constant fraction of time, and the value should be stable across harts (RISC-V lingo for physical CPU thread).
1 year ago
#define AV_READ_TIME ff_read_time
lavu/timer: specify RISC-V time unit
1 year ago
#define FF_TIMER_UNITS "ticks"
lavu/riscv: AV_READ_TIME cycle counter This uses the architected RISC-V 64-bit cycle counter from the RISC-V unprivileged instruction set. In 64-bit and 128-bit, this is a straightforward CSR read. In 32-bit mode, the 64-bit value is exposed as two CSRs, which cannot be read atomically, so a loop is necessary to detect and fix up the race condition where the bottom half wraps exactly between the two reads.
2 years ago
#endif
#endif /* AVUTIL_RISCV_TIMER_H */