[FFmpeg-devel] [PATCH] RISC-V:update ff_get_cpu_flags_riscv for RVV

[FFmpeg-devel] [PATCH] RISC-V:update ff_get_cpu_flags_riscv for RVV

[daichengrong]

From: daichengrong <daichengrong@iscas.ac.cn>

Availability of RVV and ZVBB should be determined with dl_hwcap.

As those extensions rely on vector registers, kernel vector support 
is required to save the state of context switching.

FFmpeg requires hwprobe for hardware capability detection, and cooperates 
with dl_hwcap to detect whether the kernel supports vector.

---
 libavutil/riscv/cpu.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/libavutil/riscv/cpu.c b/libavutil/riscv/cpu.c
index 163e4fc14a..fad63eccea 100644
--- a/libavutil/riscv/cpu.c
+++ b/libavutil/riscv/cpu.c
@@ -55,6 +55,10 @@ int ff_get_cpu_flags_riscv(void)
         { RISCV_HWPROBE_KEY_CPUPERF_0, 0 },
     };
 
+#if HAVE_GETAUXVAL || HAVE_ELF_AUX_INFO
+    const unsigned long hwcap = ff_getauxval(AT_HWCAP);
+#endif
+
     if (__riscv_hwprobe(pairs, FF_ARRAY_ELEMS(pairs), 0, NULL, 0) == 0) {
         if (pairs[0].value & RISCV_HWPROBE_BASE_BEHAVIOR_IMA)
             ret |= AV_CPU_FLAG_RVI;
@@ -62,6 +66,12 @@ int ff_get_cpu_flags_riscv(void)
         if (pairs[1].value & RISCV_HWPROBE_IMA_V)
             ret |= AV_CPU_FLAG_RVV_I32 | AV_CPU_FLAG_RVV_I64
                  | AV_CPU_FLAG_RVV_F32 | AV_CPU_FLAG_RVV_F64;
+#if HAVE_GETAUXVAL || HAVE_ELF_AUX_INFO
+        /* The V extension implies all Zve* functional subsets */
+        if (!(hwcap & HWCAP_RV('V')))
+            ret &= ~(AV_CPU_FLAG_RVV_I32 | AV_CPU_FLAG_RVV_I64
+                 | AV_CPU_FLAG_RVV_F32 | AV_CPU_FLAG_RVV_F64);
+#endif
 #endif
 #ifdef RISCV_HWPROBE_EXT_ZBB
         if (pairs[1].value & RISCV_HWPROBE_EXT_ZBB)
@@ -76,6 +86,10 @@ int ff_get_cpu_flags_riscv(void)
 #ifdef RISCV_HWPROBE_EXT_ZVBB
         if (pairs[1].value & RISCV_HWPROBE_EXT_ZVBB)
             ret |= AV_CPU_FLAG_RV_ZVBB;
+#if HAVE_GETAUXVAL || HAVE_ELF_AUX_INFO
+        if (!(hwcap & HWCAP_RV('V')))
+            ret &= ~AV_CPU_FLAG_RV_ZVBB;
+#endif
 #endif
         switch (pairs[2].value & RISCV_HWPROBE_MISALIGNED_MASK) {
             case RISCV_HWPROBE_MISALIGNED_FAST:
-- 
2.25.1

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Re: [FFmpeg-devel] [PATCH] RISC-V:update ff_get_cpu_flags_riscv for RVV

[Rémi Denis-Courmont]

Hi,

Le 14 mars 2025 17:32:57 GMT+07:00, daichengrong@iscas.ac.cn a écrit :
>From: daichengrong <daichengrong@iscas.ac.cn>
>
>Availability of RVV and ZVBB should be determined with dl_hwcap.

No. That's completely superfluous since we already check for kernel support with hwprobe().

And we can't check for Zb* and Zv* with hwcap anyhow.

>As those extensions rely on vector registers, kernel vector support 
>is required to save the state of context switching.

No. Kernel context switching is already ascertained. And we don't care about libc context support, since vectors are clobbered by function calls, e.g. for long jumps or ucontext.
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