`jmp mpenter`.
x86-64 requires rsp to be 16-byte aligned immediately before a `call`, so a
function starts with rsp ending in 8 -- the return address -- and its
`push %rbp; mov %rsp,%rbp` prologue leaves %rbp 16-byte aligned. Compilers
build on that: an aligned vector store to a frame slot is `movaps`, and
`movaps` to a misaligned address is #GP.
entry64mp loaded the AP's stack pointer, which is page-aligned, and *jumped*.
Nothing pushed a return address, so %rbp came out eight bytes off -- on every
frame, on every application processor, for the life of the machine.
It cost nothing for as long as no function an AP ran needed an aligned vector
store. Then the console's glyph path started returning a small struct by value
(the cell-as-a-surface change), clang spilled it with movaps, and CPU 1 took a
#GP the instant it announced itself on a verbose boot:
unexpected trap 13 from cpu 1 rip ffff800000120a3b <draw_glyph_px+0x1cb>
120a37: 0f 10 45 90 movups -0x70(%rbp), %xmm0
120a3b: 0f 29 45 b0 movaps %xmm0, -0x50(%rbp) <- #GP
movups on the line above loads fine; movaps stores and faults. The
displacement is a multiple of 16, so what is misaligned is %rbp itself, which
is a fact about how the function was entered.
`call mpenter`, not `jmp`. Eight bytes for an ABI the whole toolchain assumes.
**It was hiding behind a second bug.** The first report was not a #GP at all:
Stop code ACQUIRE
#2 cprintf+0xae
#3 trap+0x2c4
trap() reported the fault with cprintf(), cprintf() takes the console lock,
and the faulting code was already holding it -- on a verbose boot every
character is drawn under that lock. acquire() panicked about a lock its own
call chain owned, and the trap number, the faulting address and the registers
were all lost. A kernel-mode trap ends in panic() a dozen lines later anyway,
so the console goes into panic mode *before* the report rather than during it.
**And a third, in the same boot.** The console reported "JetBrains Mono, 8x16
cells" -- a name from one font and metrics from another. The built-in bitmap
became a provider like any other in the previous commit, so draw_glyph_px()
installs it lazily on the first glyph; on a verbose boot that happens before
gfx_font_init() runs, which then returned early because a font was already
installed. The console spent the whole boot in the fallback while claiming not
to be. "A font is installed" and "a typeface was chosen" had been the same test
and quietly stopped being; they are two flags now.
Verified: verbose boot reaches a shell twice running with no traps and no
panics, and reports 10x21 cells -- the real typeface -- where it reported 8x16
before. Both supported configurations quiet: one boot each, no traps, no
panics, socktest and memfdtest green.
Written up as section 22 of docs/history/kernel.md, because the interesting
part is not the missing eight bytes but that three separate faults had to be
peeled off in order before the first one could be seen.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
The 32-bit entry sets EFER.SCE on its way into long mode. entry64uefi
skips that bootstrap entirely, because firmware has already done the mode
switch -- and with it skipped the one bit in there that is not about
paging. UEFI has no reason to set SCE and does not, so the kernel came up
with SYSCALL disabled.
Nothing noticed until the UEFI path first got far enough to run a user
program, at which point the first sysretq raised #UD: "unexpected trap 6
... rip <syscall_trapret>" in pid 1, at zero ticks.
Read-modify-write, because LME and LMA are already set by firmware and
have to stay.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Loads the kernel from the ESP, places its ELF segments, records the
framebuffer, translates the UEFI memory map into E820 entries, exits boot
services, builds the page tables the kernel expects and jumps. Verified
under OVMF: the kernel starts, brings up PCI, VGA, e1000 and the RTC, and
reaches "cpu0: starting".
UEFI structures are written from the specification rather than importing
gnu-efi or EDK II -- a large dependency for two hundred lines of use, and
a licensing question the project does not need to answer. clang emits
PE/COFF directly, so this needs no new toolchain.
The kernel gained a second entry point. The multiboot entry is 32-bit
code that climbs into long mode itself, and firmware that already put us
there would have to drop back down only to climb out again -- a mode
switch whose failure mode is a triple fault with no console. So entry.S
publishes its 64-bit entry the same way it publishes the multiboot
header: a magic to scan for, SCH64BOO, with include/schboot.h owning the
constants so the two cannot disagree. That header goes *after* the
multiboot header, never before: header_addr says the multiboot header
lives at mboot_load_addr, and the BIOS loader computes the kernel's file
offset from it.
The kernel loads its own GDT at that entry rather than the loader doing
it. Arriving in long mode means arriving on the firmware's descriptors,
and the first thing depending on the kernel's own selectors faulted --
"unexpected trap 13 ... cs=38", a long way from the cause.
KNOWN LIMITATION, diagnosed and not yet fixed: e820_init takes the usable
run *containing the kernel*, which under OVMF ends at 8 MB -- the
firmware places an ACPI NVS region at 0x800000, so the run is genuinely
fragmented. The map is truthful and untruncated (21 entries); the
assumption of one contiguous run is BIOS-shaped. The fix is for the
kernel to free each usable region rather than one range, which touches
kalloc and deserves its own change rather than the tail of this one.
BIOS boot is unaffected: 27 checks, no failures.
An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.
No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.
