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Map device registers with ioremap instead of assuming a low BAR

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Every driver reached its registers with P2V(bar.base), which assumes the
BAR sits inside the 0-4 GB the direct map covers. Under SeaBIOS it always
does. Under OVMF it does not: firmware may place a 64-bit BAR anywhere,
and it puts virtio-gpu's at about 768 GB. P2V produced an address with no
translation behind it, so the driver's first register write took a page
fault, then a double fault, then a triple fault and a reboot loop --
which, coming from a driver bind, reads as broken hardware rather than a
missing mapping.

ioremap() carries each region on a 1 GB page in PDPT entries 4-511, which
were unused: the direct map occupies 0-3, and kpml4[0], kpml4[KERNBASE]
and every process's PML4 all share that one table, so a mapping appears
in every address space at once with no page table to allocate. The pages
are cache-disabled and write-through for the same reason the top GB of
the direct map is -- these are registers, and a write sitting in a cache
line is a write the device never sees.

A physical address below 4 GB returns the direct-map address unchanged,
so the BIOS path, where every BAR is low, is byte for byte what it was.

virtio-gpu now initialises under OVMF (1280x800, queue 64) instead of
triple faulting. e1000 and xhci had the identical latent bug; xhci was
already refusing to bind above 4 GB with a comment saying the mapping did
not exist yet, and now it does.

BIOS unchanged: bootcheck 108 [ok] with -Fb -Tty -Usb -Sock -Evdev -Ps,
e1000 and xhci both binding.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
Nexoniarz 2026-08-09 21:30:35 +02:00
commit b768f5ded9
5 changed files with 106 additions and 12 deletions
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5
include/defs.h
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@ -437,6 +437,11 @@ void console_takeover(void);
// vm.c
void seginit(void);
void kvmalloc(void);
// Map device registers. Returns a kernel address for `size` bytes of physical
// MMIO at `pa`, or 0 if the window is exhausted. Use this rather than P2V for
// anything that came out of a BAR: firmware may place a 64-bit BAR far above
// the 4 GB the direct map covers.
void* ioremap(uint64 pa, uint64 size);
pml4e_t* setupkvm(void);
char* uva2ka(pml4e_t*, char*);
addr_t allocuvm(pml4e_t*, uint64, uint64);