perf(display): present copies window rows as-is (scanout R8G8B8X8 = window byte order, Host hands out row bytes, Framebuffer::write_rgba_row); Window copy_from/copy_to one fenced memcpy; full present ~65ms -> ~3ms

crates/drivers/framebuffer/virtio/src/lib.rs

@@ -37,6 +37,11 @@ pub struct VirtioFramebuffer {
     dirty: Option<(usize, usize, usize, usize)>,
 }
 
+/// `c` as the screen stores it: the bytes R, G, B, then one ignored.
+fn word(c: Color) -> u32 {
+    u32::from_le_bytes(c.to_rgba())
+}
+
 impl VirtioFramebuffer {
     /// Takes over the virtio GPU behind `mmio` and makes a framebuffer its
     /// scanout. `None` if it is not a GPU or any step fails.
@@ -113,6 +118,17 @@ impl Framebuffer for VirtioFramebuffer {
         self.height
     }
 
+    fn write_rgba_row(&mut self, x: ScreenCoord, y: ScreenCoord, bytes: &[u8]) {
+        let (w, h) = (self.width.raw() as usize, self.height.raw() as usize);
+        let (x, y) = (x.raw() as usize, y.raw() as usize);
+        if x >= w || y >= h {
+            return;
+        }
+        let n = (bytes.len() / FB_BPP).min(w - x);
+        self.touch(x, y, x + n, y + 1);
+        self.pixels.copy_from(self.pixel(x, y), &bytes[..n * FB_BPP]);
+    }
+
     fn write_pixel(&mut self, x: ScreenCoord, y: ScreenCoord, color: Color) {
         self.write_row(x, y, &[color]);
     }
@@ -129,7 +145,7 @@ impl Framebuffer for VirtioFramebuffer {
         let mut at = self.pixel(x, y);
         for chunk in pixels[..n].chunks(RUN) {
             for (slot, c) in run.iter_mut().zip(chunk) {
-                *slot = c.raw();
+                *slot = word(*c);
             }
             self.pixels.write_u32s(at, &run[..chunk.len()]);
             at = at + chunk.len() * FB_BPP;
@@ -144,14 +160,14 @@ impl Framebuffer for VirtioFramebuffer {
         let bottom = (y + h.raw() as usize).min(fh);
         self.touch(x, y, x + w, bottom);
         for row in y..bottom {
-            self.pixels.fill_u32(self.pixel(x, row), w, color.raw());
+            self.pixels.fill_u32(self.pixel(x, row), w, word(color));
         }
     }
 
     fn clear(&mut self, color: Color) {
         let (w, h) = (self.width.raw() as usize, self.height.raw() as usize);
         self.touch(0, 0, w, h);
-        self.pixels.fill_u32(ByteOffset::ZERO, w * h, color.raw());
+        self.pixels.fill_u32(ByteOffset::ZERO, w * h, word(color));
     }
 
     fn scroll_up(&mut self, top: ScreenCoord, height: Dimension, by: Dimension, fill: Color) {
@@ -163,7 +179,7 @@ impl Framebuffer for VirtioFramebuffer {
         self.touch(0, top, self.width.raw() as usize, bottom);
         self.pixels.copy_within(self.pixel(0, top + by), self.pixel(0, top), moved * self.stride);
         let width = self.width.raw() as usize;
-        self.pixels.fill_u32(self.pixel(0, bottom - by), by * width, fill.raw());
+        self.pixels.fill_u32(self.pixel(0, bottom - by), by * width, word(fill));
     }
 
     /// Sends the host only what changed since the last flush.