hw/riscv/iommu: fix build error with clang

Introduced in 0c54acb8243, "hw/riscv: add RISC-V IOMMU base emulation".

../hw/riscv/riscv-iommu.c:187:17: error: redefinition of '_pext_u64'

  187 | static uint64_t _pext_u64(uint64_t val, uint64_t ext)

      |                 ^

D:/a/_temp/msys64/clang64/lib/clang/18/include/bmi2intrin.h:217:1: note: previous definition is here

  217 | _pext_u64(unsigned long long __X, unsigned long long __Y)

      | ^

After a conversation on the mailing list, it was decided to rename and
add a comment for this function.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241104222225.1523751-1-pierrick.bouvier@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

hw/riscv/riscv-iommu.c

@@ -183,8 +183,25 @@ static void riscv_iommu_pri(RISCVIOMMUState *s,
     }
 }
 
-/* Portable implementation of pext_u64, bit-mask extraction. */
-static uint64_t _pext_u64(uint64_t val, uint64_t ext)
+/*
+ * Discards all bits from 'val' whose matching bits in the same
+ * positions in the mask 'ext' are zeros, and packs the remaining
+ * bits from 'val' contiguously at the least-significant end of the
+ * result, keeping the same bit order as 'val' and filling any
+ * other bits at the most-significant end of the result with zeros.
+ *
+ * For example, for the following 'val' and 'ext', the return 'ret'
+ * will be:
+ *
+ * val = a b c d e f g h
+ * ext = 1 0 1 0 0 1 1 0
+ * ret = 0 0 0 0 a c f g
+ *
+ * This function, taken from the riscv-iommu 1.0 spec, section 2.3.3
+ * "Process to translate addresses of MSIs", is similar to bit manip
+ * function PEXT (Parallel bits extract) from x86.
+ */
+static uint64_t riscv_iommu_pext_u64(uint64_t val, uint64_t ext)
 {
     uint64_t ret = 0;
     uint64_t rot = 1;
@@ -528,7 +545,7 @@ static MemTxResult riscv_iommu_msi_write(RISCVIOMMUState *s,
     int cause;
 
     /* Interrupt File Number */
-    intn = _pext_u64(PPN_DOWN(gpa), ctx->msi_addr_mask);
+    intn = riscv_iommu_pext_u64(PPN_DOWN(gpa), ctx->msi_addr_mask);
     if (intn >= 256) {
         /* Interrupt file number out of range */
         res = MEMTX_ACCESS_ERROR;