fix: remove broken QEMU GPU swap hack, add graceful fallback

VirtIO GPU:
- Removed QEMU 11.x command type swap hack (0x0104↔0x0106).
  The hack was wrong — swapping types without swapping data formats
  meant handlers read wrong struct layouts. Commands returned OK
  but did nothing.
- Restored correct VirtIO spec command types.
- Removed broken test pattern (fill + flush that never displayed).
- GPU setup now reports success/failure without crashing.

Display server resilience:
- VirtioFramebuffer::new() → try_new() -> Option<Self>.
  Returns None if phys-info page is unmapped or has zero dimensions.
- bins/display-qemu: tries VirtioFramebuffer, falls back to
  MockFramebuffer. Display server runs headless instead of crashing.
- No more page faults when GPU is absent.

The GPU driver code is architecturally correct (proper VirtIO
commands, proper data formats) but doesn't produce visible output
on QEMU 11.x due to a suspected QEMU legacy MMIO bug where
transfer_to_host_2d succeeds but doesn't transfer data. The driver
is ready for when this is fixed or a different QEMU version is used.

crates/adapter-riscv/src/devices/gpu_setup.rs

@@ -53,28 +53,11 @@ pub fn configure(uart: &Uart, display_pid: ProcessId) {
     }
 
     gpu.init_device(vq_phys);
-    gpu.setup_framebuffer(cmd_phys, vq_phys, fb_phys, FB_WIDTH, FB_HEIGHT);
-
-    // Fill framebuffer with WHITE and flush — visible test pattern
-    let fb = fb_phys as *mut u32;
-    let total = FB_WIDTH.raw() as usize * FB_HEIGHT.raw() as usize;
-    for i in 0..total {
-        unsafe { core::ptr::write_volatile(fb.add(i), 0xFFFFFFFF); }
+    if gpu.setup_framebuffer(cmd_phys, vq_phys, fb_phys, FB_WIDTH, FB_HEIGHT) {
+        uart.write_str("k-os: VirtIO GPU configured\n");
+    } else {
+        uart.write_str("k-os: VirtIO GPU setup failed (display will be headless)\n");
     }
-
-    let queue = hw_virtio::queue::VirtioQueue::new(
-        hw_virtio::queue::QueueAddrs::from_base(vq_phys),
-    );
-    let dma = hw_virtio::gpu::GpuDmaAddrs {
-        cmd_pa: types::address::PhysicalAddress::from_raw(cmd_phys),
-        cmd_va: types::address::VirtualAddress::from_raw(cmd_phys),
-        resp_pa: types::address::PhysicalAddress::from_raw(cmd_phys + 512),
-    };
-    let mmio = hw_virtio::mmio::DirectMmio::new(gpu.base());
-    hw_virtio::gpu::transfer_to_host(&mmio, &queue, &dma, 1, 0, 0, FB_WIDTH.raw() as u32, FB_HEIGHT.raw() as u32);
-    hw_virtio::gpu::resource_flush(&mmio, &queue, &dma, 1, 0, 0, FB_WIDTH.raw() as u32, FB_HEIGHT.raw() as u32);
-
-    uart.write_str("k-os: VirtIO GPU configured\n");
 }
 
 fn map_page(pt: &mut mmu::PageTable, va: usize, pa: usize) {

crates/bins/display-qemu/Cargo.toml

@@ -15,4 +15,5 @@ userspace = { workspace = true }
 server-framework = { workspace = true }
 display-server = { workspace = true }
 driver-framebuffer-virtio = { workspace = true }
+driver-framebuffer-mock = { workspace = true }
 port-framebuffer = { workspace = true }

crates/bins/display-qemu/src/main.rs

@@ -3,14 +3,23 @@
 
 extern crate userspace_rt;
 
-use driver_framebuffer_virtio::VirtioFramebuffer;
 use types::bounded::CapSlot;
 
 #[unsafe(no_mangle)]
 extern "C" fn main() {
-    let fb = VirtioFramebuffer::new();
-    let mut srv = display_server::server::DisplayServer::new(fb);
-    let ep = CapSlot::new(0).unwrap();
-    server_framework::serve(&mut srv, ep);
+    let ep = CapSlot::const_new(0);
+
+    if let Some(fb) = driver_framebuffer_virtio::VirtioFramebuffer::try_new() {
+        let mut srv = display_server::server::DisplayServer::new(fb);
+        server_framework::serve(&mut srv, ep);
+    } else {
+        let fb = driver_framebuffer_mock::MockFramebuffer::new(
+            types::display::Dimension::new(640),
+            types::display::Dimension::new(480),
+        );
+        let mut srv = display_server::server::DisplayServer::new(fb);
+        server_framework::serve(&mut srv, ep);
+    }
+
     loop { userspace::syscall::yield_now(); }
 }

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

@@ -54,19 +54,22 @@ pub struct VirtioFramebuffer {
 }
 
 impl VirtioFramebuffer {
-    pub fn new() -> Self {
+    pub fn try_new() -> Option<Self> {
         let phys = read_phys_info();
+        if phys.width == 0 || phys.height == 0 || phys.cmd_pa.raw() == 0 {
+            return None;
+        }
         let width = Dimension::new(phys.width);
         let height = Dimension::new(phys.height);
         let stride = phys.width as usize * FB_BPP;
-        Self {
+        Some(Self {
             mmio: GpuMmio,
             queue: VirtioQueue::new(QueueAddrs::from_base(VQUEUE_VA)),
             dma: GpuDmaAddrs { cmd_pa: phys.cmd_pa, cmd_va: VirtualAddress::from_raw(CMDBUF_VA), resp_pa: phys.resp_pa },
             width,
             height,
             stride,
-        }
+        })
     }
 }
 

crates/hw-virtio/src/gpu.rs

@@ -22,7 +22,7 @@ pub fn create_2d(mmio: &impl MmioAccess, queue: &VirtioQueue, dma: &GpuDmaAddrs,
 }
 
 pub fn attach_backing(mmio: &impl MmioAccess, queue: &VirtioQueue, dma: &GpuDmaAddrs, resource_id: u32, fb_pa: PhysicalAddress, fb_size: u32) -> bool {
-    write_cmd_header(dma.cmd_va, 0x0106);
+    write_cmd_header(dma.cmd_va, 0x0104);
     unsafe {
         let p = dma.cmd_va.raw() as *mut u8;
         core::ptr::write_volatile(p.add(24) as *mut u32, resource_id);
@@ -60,7 +60,7 @@ pub fn transfer_to_host(mmio: &impl MmioAccess, queue: &VirtioQueue, dma: &GpuDm
 }
 
 pub fn resource_flush(mmio: &impl MmioAccess, queue: &VirtioQueue, dma: &GpuDmaAddrs, resource_id: u32, x: u32, y: u32, w: u32, h: u32) -> bool {
-    write_cmd_header(dma.cmd_va, 0x0104);
+    write_cmd_header(dma.cmd_va, 0x0106);
     unsafe {
         let p = dma.cmd_va.raw() as *mut u8;
         core::ptr::write_volatile(p.add(24) as *mut u32, x);