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dma: the 4 MB cap was the refusal, not the arena

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vgpud: wanted 7365 KB of DMA memory for a 1920x982 framebuffer
    vgpud: not enough contiguous DMA memory
    schinitd: restarting os.schos.vgpud        (and again, and again, until
    schinit: supervisor stopped answering       schinit replaced schinitd)

Every number in that diagnostic was about the arena, and the arena had 61 MB
free. The refusal came from do_dma()'s per-request cap, which was 4 MB --
exactly one framebuffer at 1280x800, chosen when that was the only thing
anybody asked for.

virtio-gpu's display size is the size of the **host's window**. Drag it out and
the guest is asked for 1920x982x4 in one contiguous piece, and no amount of
free arena helps. 32 MB now, which is 2560x1600 with room over. The cap is not
there to ration memory -- the arena does that and says so properly -- it is
there so a wild size cannot ask for everything by accident.

DMA_ARENA_BYTES 16 -> 64 MB alongside, for the reason its own comment already
gave and then got the arithmetic wrong on: either side of the handover *both*
drivers hold a framebuffer, so the requirement is two at the largest mode, not
one. 16 MB is two at 1280x800 and nothing bigger. Verified booting at 256M and
512M as well as 1G.

And vgpud no longer dies when it cannot have what it asked for. It claims the
framebuffer before creating the resource, and if the full size cannot be had it
steps down by thirds to 640x480 before giving up -- and gives up with
SCHINIT_EXIT_NORESTART rather than die(), because a driver that will fail
identically on every restart should not be restarted. That loop is what made
schinit replace the supervisor mid-boot.

    vgpud: the display is 1920x1080 but only 1280x720 of DMA could be had

is what it says when it steps down. With the cap raised it does not need to:
1920x1080 now allocates its full 8100 KB, 0 restarts.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
This commit is contained in:
Nexoniarz 2026-09-14 22:52:20 +02:00
commit ca28978995
4 changed files with 121 additions and 34 deletions
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84
cmd/vgpud/main.c
View file

@ -418,6 +418,66 @@ bring_up(void)
vputs("vgpud: display is "); vputn(width); vputs("x"); vputn(height); vputs("\n");
// The framebuffer is claimed **before** the resource is created, because
// the resource has to be created at a size we can actually back.
//
// virtio-gpu's display size is the size of the host's window, which the
// user drags to whatever they like -- and every pixel of it is four bytes
// of contiguous DMA that has to be found twice during the handover, once
// for the kernel driver and once for this one. A big enough window does
// not fit, and what that used to look like was not a message, it was a
// loop:
//
// vgpud: wanted 7365 KB of DMA memory for a 1920x982 framebuffer
// schinitd: restarting os.schos.vgpud
// vgpud: wanted 7365 KB ... (and again, and again, until
// schinit: supervisor stopped answering schinit replaced schinitd)
//
// So: ask for what the display says, and if that cannot be had, ask for
// less. A smaller scanout on a larger display is a worse picture; a driver
// that will not start is no picture and a supervisor fighting itself.
{
struct pciuser_dma d;
int w = width, h = height, tries;
for (tries = 0; ; tries++) {
d.size = (unsigned long long)w * h * 4;
if (ioctl(pcifd, PCIIOC_DMA, &d) >= 0)
break;
// Two thirds each time, on an eight-pixel grid. Not halving:
// halving overshoots badly, and the arena is usually short by a
// little rather than by a lot.
w = ((w * 2 / 3) + 7) & ~7;
h = ((h * 2 / 3) + 7) & ~7;
if (w < 640 || h < 480) {
puts("vgpud: wanted "); putn(((long long)width * height * 4) / 1024);
puts(" KB of DMA memory for a "); putn(width); puts("x");
putn(height); puts(" framebuffer, and nothing down to 640x480\n");
puts("vgpud: `schdbg dma` shows the arena -- this is "
"DMA_ARENA_BYTES, not fragmentation\n");
// Not die(): a died driver is restarted, and restarting will
// fail identically for ever. The kernel keeps the VBE console,
// which is a working screen.
nothing_to_do("no room for a framebuffer; the console keeps the "
"VBE framebuffer");
}
}
if (tries) {
puts("vgpud: the display is "); putn(width); puts("x"); putn(height);
puts(" but only "); putn(w); puts("x"); putn(h);
puts(" of DMA could be had; using that\n");
width = w;
height = h;
}
fb_uaddr = d.uaddr;
fb_paddr = d.paddr;
}
fb_bytes = (unsigned int)(width * height * 4);
max_w = width;
max_h = height;
// A 2D resource the size of the screen.
struct virtio_gpu_resource_create_2d *c =
(struct virtio_gpu_resource_create_2d *)cmdbuf;
@ -429,29 +489,7 @@ bring_up(void)
if (docmd(sizeof(*c), sizeof(struct virtio_gpu_ctrl_hdr)) < 0 || !resp_ok())
die("RESOURCE_CREATE_2D refused");
// Memory behind it. This is the framebuffer: the process writes here and
// the device reads it, which is the entire reason PCIIOC_DMA reports a
// physical address.
fb_bytes = (unsigned int)(width * height * 4);
struct pciuser_dma d;
d.size = fb_bytes;
if (ioctl(pcifd, PCIIOC_DMA, &d) < 0) {
// The kernel has already printed how much is free and how big the
// largest piece is, which is the part that says whether this is a full
// arena or a fragmented one. All this adds is what was being asked
// for, and the resolution that asked for it -- because the usual cause
// is a mode nobody realised was this expensive.
puts("vgpud: wanted "); putn(fb_bytes / 1024);
puts(" KB of DMA memory for a "); putn(width); puts("x"); putn(height);
puts(" framebuffer\n");
puts("vgpud: `schdbg dma` shows the arena; a smaller mode needs less\n");
die("not enough contiguous DMA memory");
}
fb_uaddr = d.uaddr;
fb_paddr = d.paddr;
max_w = width;
max_h = height;
// The framebuffer was claimed above, before the resource was created.
struct virtio_gpu_resource_attach_backing *a =
(struct virtio_gpu_resource_attach_backing *)cmdbuf;