fix: use MMIO-safe copy for goldfish gralloc writes and prefer RW mmap

The AVC encode test crashed with SIGSEGV when writing to goldfish
address space memory via Go's copy(), which emits AVX2 VMOVDQU
instructions that fault on uncacheable PCI BAR (MMIO) memory.

Two fixes:
- Add CopyToMMIO for scalar writes to MMIO regions (matching the
  existing copyFromMMIO for reads)
- Reorder mmap strategies to try PROT_READ|PROT_WRITE before
  PROT_READ, so buffers are writable when the kernel allows it

Author: xaionaro@dx.center

gralloc/buffer.go

@@ -66,10 +66,10 @@ type mmapAttempt struct {
 //   - Some gralloc FDs require PROT_READ|PROT_WRITE
 //   - Some FDs only support MAP_PRIVATE
 var mmapStrategies = []mmapAttempt{
-	{unix.PROT_READ, unix.MAP_SHARED, "PROT_READ|MAP_SHARED"},
 	{unix.PROT_READ | unix.PROT_WRITE, unix.MAP_SHARED, "PROT_READ|PROT_WRITE|MAP_SHARED"},
-	{unix.PROT_READ, unix.MAP_PRIVATE, "PROT_READ|MAP_PRIVATE"},
+	{unix.PROT_READ, unix.MAP_SHARED, "PROT_READ|MAP_SHARED"},
 	{unix.PROT_READ | unix.PROT_WRITE, unix.MAP_PRIVATE, "PROT_READ|PROT_WRITE|MAP_PRIVATE"},
+	{unix.PROT_READ, unix.MAP_PRIVATE, "PROT_READ|MAP_PRIVATE"},
 }
 
 // DMA-BUF sync ioctl constants. On kernel 6.6+ the DMA-BUF subsystem
@@ -340,6 +340,30 @@ func copyFromMMIO(src []byte) []byte {
 	return dst
 }
 
+// CopyToMMIO copies src into a goldfish address space mmap region dst.
+// Like copyFromMMIO, this avoids Go's runtime.memmove which uses AVX2
+// VMOVDQU instructions that crash on UC (uncacheable) PCI BAR memory.
+// Only min(len(src), len(dst)) bytes are copied; the count is returned.
+//
+//go:noinline
+func CopyToMMIO(dst []byte, src []byte) int {
+	n := len(src)
+	if len(dst) < n {
+		n = len(dst)
+	}
+
+	i := 0
+	for ; i+8 <= n; i += 8 {
+		v := *(*uint64)(unsafe.Pointer(&src[i]))
+		*(*uint64)(unsafe.Pointer(&dst[i])) = v
+	}
+	for ; i < n; i++ {
+		dst[i] = src[i]
+	}
+
+	return n
+}
+
 // isGoldfishFD checks if an FD points to the goldfish emulator's
 // address space device.
 func isGoldfishFD(fd int) bool {

tests/e2e/codec2_test.go

@@ -914,12 +914,11 @@ func testCodec2HIDL_EncodeAVC(t *testing.T) {
 		t.Logf("AVC gralloc mmap failed (expected on goldfish): %v", mmapErr)
 	} else {
 		grayFrame := makeGrayYUVFrame(encWidth, encHeight)
-		copyLen := len(grayFrame)
-		if copyLen > len(buf.MmapData) {
-			copyLen = len(buf.MmapData)
-		}
-		copy(buf.MmapData[:copyLen], grayFrame[:copyLen])
-		t.Logf("AVC: wrote %d bytes of gray YUV to gralloc buffer", copyLen)
+		// Use CopyToMMIO instead of copy(): goldfish address space is
+		// backed by a PCI BAR (UC MMIO), and Go's copy() emits AVX2
+		// VMOVDQU instructions that cause SIGSEGV on UC memory.
+		n := gralloc.CopyToMMIO(buf.MmapData, grayFrame)
+		t.Logf("AVC: wrote %d bytes of gray YUV to gralloc buffer", n)
 	}
 
 	// Register listener and create component.