gfx: the second head shows the boot, not just what comes after it

Two things, both asked for.

**The crash report no longer says which monitors it reached.** "The report is
on the virtio-gpu display too" was a line about the crash screen printed on the
crash screen. The report says what happened and where to read it again; that is
not part of it.

**And the second head now shows the F8 menu and the boot.** It used to be black
until vgpud started several seconds into userspace -- through the splash,
through the menu, through every boot message -- which from the outside is
indistinguishable from a display that does not work. Three stages now cover the
whole boot and hand over to each other without coordinating:

*The loader.* LocateProtocol() answers with the first GOP and offers no way to
ask for the rest, so the rest are found by handle and fed the primary's
framebuffer as a Blt source at the points where a screen is finished: the menu,
the resolution chooser, system information, the splash, each segment of the
kernel load. Blt rather than a second framebuffer to draw into, because a
virtio GOP does not have one -- the same fact that made a virtio-only machine
reboot-loop two commits ago.

*The kernel.* gfx remembers which rows were drawn, as one span rather than a
list of rectangles, because console output is lines and a span is what lines
make. display_mirror_console() copies that span across: a line of boot text is
one cell-row transfer, not a screen. Primed once after pci_init(), because a
display that has just appeared has a black scanout and the console it should be
showing was drawn by the loader into a framebuffer it knows nothing about.

*Then vgpud*, from the same console framebuffer, unchanged.

**Two bugs found building it, both worth the retelling.**

The mirror ran per character from inside vt_feed -- which is inside the console
lock -- and vga_disp_acquire() announces its back buffer with a cprintf. A CPU
taking a lock it already holds, with a call chain that said exactly that. It
runs at the end of consolewrite() and cprintf() now, outside the lock, which is
also the better batching: one transfer per write instead of one per character.

And "is this the console's own screen" was asked of the pointer acquire()
returned. vga_fb hands back a back buffer it copies onto the console's
framebuffer afterwards, so on a one-screen machine the answer was "a different
screen" -- and the console was mirrored onto itself through two megabytes of
intermediate, allocated for the purpose. display_ops grew scanout_phys, which
is a fact about the hardware rather than a policy a driver can get subtly
wrong, and it is asked first: it has no side effects and acquire() has two.

Verified with two heads under QEMU, screendumping the virtio head every 400 ms
from power-on: splash, splash, splash, then the console, and it stays. The F8
menu appears on it; choosing 1024x768 there brings both heads up at 1024x768
byte-identical. A panic still reaches both. Both supported configurations: one
boot each, no traps, no panics, socktest and memfdtest green.

One gap remains and is documented rather than hidden: while the kernel's driver
resets the device during probe, QEMU shows "Display output is not active" for a
fraction of a second. A device being taken over has no scanout until the new
driver sets one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

kernel/drivers/gpu/vga/vga_fb.c

@@ -225,11 +225,22 @@ vga_disp_wait_vblank(struct device *d)
       break;
 }
 
+// Whatever acquire() handed out, the pixels end up here: the linear
+// framebuffer this driver published as the console's scanout. Saying so is
+// what keeps the console mirror from copying the console onto itself.
+static uint64
+vga_disp_scanout_phys(struct device *d)
+{
+  (void)d;
+  return fb_phys;
+}
+
 const struct display_ops vga_display_ops = {
   .mode        = vga_disp_mode,
   .acquire     = vga_disp_acquire,
   .present     = vga_disp_present,
   .wait_vblank = vga_disp_wait_vblank,
+  .scanout_phys = vga_disp_scanout_phys,
 };
 
 // Put the VBE framebuffer back on screen.

kernel/drivers/gpu/virtio_gpu/virtio_gpu.c

@@ -402,11 +402,24 @@ vgpu_present(struct device *d, int x, int y, int w, int h)
 // No wait_vblank. The device presents whole rectangles atomically, so there is
 // nothing to synchronise against -- and a null op is more honest than one that
 // spins and achieves nothing. display_wait_vblank() handles the absence.
+// The guest-side framebuffer, which is this device's own memory and not the
+// console's -- on every machine this driver has run on. It is written as a
+// comparison rather than as `return 0` because a machine whose *only* display
+// is a virtio-gpu would have the console here, and then the honest answer
+// changes without this line needing to.
+static uint64
+vgpu_scanout_phys(struct device *d)
+{
+  struct vgpu *g = vgpu_of(d);
+  return (g && g->fb) ? dma_phys(g->fb) : 0;
+}
+
 const struct display_ops virtio_gpu_display_ops = {
   .mode        = vgpu_mode,
   .acquire     = vgpu_acquire,
   .present     = vgpu_present,
   .wait_vblank = 0,
+  .scanout_phys = vgpu_scanout_phys,
 };
 
 // --- probe -----------------------------------------------------------------