d85274dbd9
Gate 0's last-but-one open item. Drop a TrueType file at \schos\console.ttf on
the ESP and the console is drawn with it; with no file, the face linked into
the kernel image, which is what an early panic needs and is not a degraded
mode.
**The loader reads it, and that is the decision worth writing down.** The
kernel opening the file once the root filesystem is mounted is the obvious
design. It is also wrong: the console has been drawing since the kernel's
first line, so a font arriving later changes the cell size, which changes the
column count, which re-lays out every terminal -- and blanks them. The kernel
log would vanish off the screen at the moment the root filesystem mounted,
which is exactly the log a machine being brought up for the first time is
being read for. Since the next thing on the list is real hardware, losing that
was not a trade worth making.
So the choice happens before anything is drawn. Three details make it cheap:
- **EfiReservedMemoryType**, not EfiLoaderData. The E820 handoff turns
loader data into usable memory, on the argument that the loader's own
pages are the kernel's the moment it runs -- true of the loader, not true
of a buffer it wants the kernel to keep reading from.
- **Below 4 GB**, because the kernel reaches this through its direct map and
that is how far the direct map goes. A font above it would need an ioremap
the console cannot do before there is a console.
- **Used in place.** No copy, which matters because the kernel has no
contiguous allocator to copy 112 KB into -- the only one is the DMA arena,
and that belongs to devices.
Sized without EFI_FILE_INFO and its GUID: seek past the end and ask where that
landed, which the specification requires to clamp.
Every failure falls back and keeps going -- no file, implausible size, no
memory, short read, unparseable file. The last says so in the log, because a
font that silently did not load looks exactly like a font that did.
Verified by booting with JetBrainsMono-ExtraBold.ttf on the ESP and
screendumping the console: visibly the bold face, against the Regular of a
boot with no file. Three boots each way, no traps and no panics, socktest and
memfdtest green in all six.
Nothing ships a console.ttf: the file would be the file already in the kernel,
and two copies of a font are two things that can disagree.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
125 lines
6 KiB
C
125 lines
6 KiB
C
// SPDX-License-Identifier: Apache-2.0
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#pragma once
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#ifndef SCHBOOT_H
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#define SCHBOOT_H
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// The loader/kernel hand-off, in one place.
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//
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// Included by kernel/arch/x86_64/entry.S and by boot/uefi/main.c, so the two
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// cannot disagree about the magic or the layout. Defining it twice is exactly
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// how a loader ends up scanning for bytes the kernel never wrote — the first
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// attempt here searched for "SCH6" while the assembler had stored 0x53434836,
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// which lands in the file as "6HCS".
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//
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// Keep this header assembler-safe: no typedefs, no structs outside the C
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// guard, nothing entry.S cannot preprocess.
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// Scanned for in the first 16 KB of the kernel image, 8-byte aligned, exactly
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// as the BIOS loader scans for the multiboot magic. Followed by a little-endian
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// 64-bit physical address: the kernel's 64-bit entry point.
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//
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// The values spell "SCH6" "4BOO" when stored little-endian, which is what
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// makes them findable by eye in a hex dump of the image.
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#define SCH64_MAGIC0 0x36484353 /* "SCH6" */
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#define SCH64_MAGIC1 0x4f4f4234 /* "4BOO" */
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// ---------------------------------------------------------------------------
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// The hand-off block.
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//
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// One page of low memory that a loader fills in and the kernel reads. Every
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// field below was previously #defined in whichever file happened to need it --
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// five of them, across two loaders and two kernel files -- which is the same
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// hazard as defining the magic twice, only quieter: nothing breaks until one
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// copy moves.
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//
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// 0x30000 is clear of stage2 (loaded at 0x7E00 and well under 0xA000), the
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// multiboot kernel-search buffer at 0x10000-0x14000, and the kernel's own load
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// address at 0x100000.
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//
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// Both loaders write it, so a field's meaning must not depend on which one did.
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// Where they genuinely differ -- the BIOS path knows what medium it booted
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// from, UEFI does not -- the field is defined as "0 when unknown" rather than
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// left for the reader to guess.
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#define SCHBOOT_BASE 0x30000
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// A VBE mode info block, 256 bytes. The UEFI loader fills the same four fields
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// out of GOP rather than translating a structure it never saw, so the kernel
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// cannot tell which firmware booted it.
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#define SCHBOOT_VBE_INFO (SCHBOOT_BASE + 0x000)
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#define SCHBOOT_VBE_STRIDE (SCHBOOT_VBE_INFO + 16) /* uint16, bytes/line */
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#define SCHBOOT_VBE_WIDTH (SCHBOOT_VBE_INFO + 18) /* uint16 */
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#define SCHBOOT_VBE_HEIGHT (SCHBOOT_VBE_INFO + 20) /* uint16 */
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#define SCHBOOT_VBE_BPP (SCHBOOT_VBE_INFO + 25) /* uint8 */
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#define SCHBOOT_VBE_LFB (SCHBOOT_VBE_INFO + 40) /* uint32, physical */
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// Non-zero if the loader was asked for a verbose boot.
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#define SCHBOOT_VERBOSE_FLAG (SCHBOOT_BASE + 0x100)
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// What the system booted from. SCHBOOT_MEDIA_CD matters because the ISO path
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// reads through ATAPI rather than ATA; 0 means "not stated", which is what the
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// UEFI loader leaves, since it does not read the kernel through either.
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#define SCHBOOT_MEDIA_FLAG (SCHBOOT_BASE + 0x101)
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#define SCHBOOT_MEDIA_CD 1
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// Real-mode spelling of the same byte, for cdboot.S which has no 32-bit
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// addressing yet. Segment and offset, not a flat address.
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#define SCHBOOT_MEDIA_SEG 0x3000
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#define SCHBOOT_MEDIA_OFF 0x101
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// The E820 memory map: a uint32 count, then that many 24-byte entries. Capped
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// at 32 so a pathological firmware cannot run past the buffer; the map ends at
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// 0x508, which is what leaves 0x600 free below.
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#define SCHBOOT_E820_COUNT (SCHBOOT_BASE + 0x200)
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#define SCHBOOT_E820_MAP (SCHBOOT_BASE + 0x208)
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/* 40, not 32. The map lives at SCHBOOT_E820_MAP (BASE+0x208) and the next
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* field is SCHBOOT_RSDP_ADDR at BASE+0x600, so there are 1016 bytes and each
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* entry is 24 -- 42 fit. Real UEFI firmware produces far more descriptors than
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* QEMU's, and the headroom costs nothing that was being used. */
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#define SCHBOOT_E820_MAX 40
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// Where the loader leaves the ACPI RSDP, as a little-endian 64-bit physical
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// address. Zero means "not provided, go and look for it yourself".
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//
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// The kernel's own search scans the EBDA and 0xE0000-0x100000, which is a BIOS
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// convention. UEFI publishes the RSDP in its configuration table instead and is
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// under no obligation to leave a copy anywhere in particular; OVMF does not.
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// Without it the kernel finds no MADT, so no IOAPIC, so no interrupt routing --
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// the machine boots to a prompt on polled serial output and then ignores every
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// key, which looks like a broken console rather than a missing table.
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//
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#define SCHBOOT_RSDP_ADDR (SCHBOOT_BASE + 0x600)
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// A console typeface, read off the boot volume by the loader.
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//
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// Zero length means "there was none" and the kernel uses the one linked into
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// its own image. That fallback is not a degraded mode: an early panic has to
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// be readable, and the only font guaranteed to exist before any filesystem
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// does is the one in the image.
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//
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// **Loaded by the loader rather than by the kernel**, which is the whole
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// design decision here. The kernel could read the file itself once the root
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// filesystem is mounted -- but the console is drawing before that, so
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// switching then means re-measuring the cell and re-laying out every terminal
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// mid-boot. That wipes the kernel log off the screen at exactly the moment a
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// machine being brought up for the first time needs it. Choosing the font
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// before the first line is drawn costs one file read and changes nothing
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// afterwards.
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//
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// The pages are allocated as EfiReservedMemoryType, so the E820 handoff marks
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// them reserved and the page allocator never hands them out -- the font is
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// used *in place*, and nothing in the kernel has to find room for 100 KB of
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// contiguous memory.
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#define SCHBOOT_FONT_ADDR (SCHBOOT_BASE + 0x608) /* uint64, physical */
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#define SCHBOOT_FONT_LEN (SCHBOOT_BASE + 0x610) /* uint32, bytes */
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#ifndef __ASSEMBLER__
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struct sch64_header {
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unsigned int magic0; // SCH64_MAGIC0
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unsigned int magic1; // SCH64_MAGIC1
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unsigned long long entry; // physical address, entered in long mode
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};
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#endif /* !__ASSEMBLER__ */
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#endif /* SCHBOOT_H */
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