651899220c
1449 lines of boot/bios/, tools/sign.pl and tools/padstage2.pl, plus the build rules, the disk image and the ISO entry that existed to carry them. Everything since roughly 2012 boots UEFI, and CSM -- the firmware-side BIOS emulation that lets a legacy loader run on such a machine at all -- has been going away since Intel dropped support for it in 2020. So this is not a path being retired early; it is one whose remaining hardware is already the exception. The cost was never the code, it was the tax on everything near it. Stage 2 had a 65-sector ceiling it overflowed silently -- 25440 bytes against 24576 -- and because the kernel is written at sector 1 + STAGE2_SECTORS, an overflowing loader was overwritten by the very kernel it was loading. The resolution list could not be shared with UEFI either: VBE mode setting is INT 10h, so matching it needed unreal mode, which is the riskiest code in the tree for a feature that already worked elsewhere. What replaces schos.img is the staged ESP, handed to QEMU as a virtual FAT volume. That is a development arrangement and not an installable disk; a real GPT image with a real FAT partition is the installer's job, and until it exists nothing here writes a disk a machine boots unaided. Secure Boot is untouched too -- the loader is unsigned. The ISO keeps one El Torito entry, the FAT image holding loader and kernel. It never booted from a USB stick via BIOS anyway: the firmware presents the stick as a hard disk while stage2main.c read the kernel back over ATAPI. The 32-bit multiboot entry in entry.S now has no caller and is left in place. Deleting kernel entry code is its own change with its own way of failing silently, and it should not ride along with this one. Ported with it: run.ps1, the six screenshot tools, and schcom_test.sh, each with its own OVMF varstore copy so no run inherits another's UEFI variables. Two things this shook out. OVMF now belongs in `all`: nothing boots without firmware, `make clean` wipes build/ including the copy fetched from the host, and a clean followed by a build left every run script throwing. And it cannot be listed as a prerequisite at the top of the file -- make expands prerequisites when it reads the line, and $(OVMF_CODE) is defined further down, so naming it there appends nothing and says nothing. Verified from a clean tree: 110 [ok] and no failures on the full suite, the ISO boots with no hard disk attached, schfs-check passes, schcom's end-to-end pixel test passes, and fbshot captures a real 1280x800 frame.
29 lines
1.4 KiB
Markdown
29 lines
1.4 KiB
Markdown
<!-- SPDX-License-Identifier: Apache-2.0 -->
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# UEFI loader
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The loader. This directory once existed only so the BIOS path could be moved
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into `boot/bios/` without pretending the split was hypothetical; it is now the
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only path there is.
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See `../README.md` for the hand-off contract a loader has to satisfy. The
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short version: the kernel wants long mode with paging on, its ELF segments
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placed, a 24-byte-entry E820-shaped memory map, and framebuffer geometry — and
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it does not care which firmware produced them.
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What this will need, in rough order:
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1. A PE/COFF target. clang can emit one directly
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(`--target=x86_64-unknown-windows -fuse-ld=lld-link`), so no new toolchain:
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the same LLVM install already builds every other target here.
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2. The handful of UEFI structures actually used — the system table, boot
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services, `LOADED_IMAGE`, `SIMPLE_FILE_SYSTEM`, `GRAPHICS_OUTPUT`. Written
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from the specification rather than by importing gnu-efi or EDK II, which is
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both a licence question and a large dependency for something this small.
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3. `GetMemoryMap` translated into the same 24-byte entries `e820.c` already
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parses. Not a second format for the kernel to learn.
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4. `ExitBootServices` before touching the memory it described.
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5. Graphics Output Protocol for the framebuffer, filling the same
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`VBE_INFO_ADDR` structure the kernel reads.
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The image goes at `EFI/BOOT/BOOTX64.EFI` on a FAT ESP, which also means the
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ISO carries a single El Torito EFI entry.
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