commit 87e4d9e6ab49713018d74732bb248f99078fbfb1
parent 40884996414a0b5bacdf306a98531f55cebdfbfe
Author: Brian Swetland <swetland@frotz.net>
Date: Sun, 29 Dec 2013 21:43:57 -0800
notes on work in progress
Diffstat:
| A | README.64BIT | | | 70 | ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ |
1 file changed, 70 insertions(+), 0 deletions(-)
diff --git a/README.64BIT b/README.64BIT
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+
+While porting Xv6 to 64bit Intel/AMD platforms, the goal is to keep changes
+to the codebase as non-intrusive as possible, leaving it as short, clean,
+and readable as it was in the original 32bit version. When possible,
+#ifdefs will be avoided, and the existing style will be maintained. Also,
+it should continue to be possible to build, boot, and run Xv6 on 32bit
+platforms from the same codebase.
+
+
+DONE
+ * Quick hack-and-slash modifications to get a buildable 64bit kernel.elf
+ * stubbed out or quickly patched up assembly glue where it didn't build
+ * temporarily turned off int-to-pointer and pointer-to-int warnings
+ to make finding build breakages faster
+ * chase down link errors due to too-large relocations
+ * switch to cross-x86-64 compiler to avoid weirdness from host compiler
+ * https://github.com/travisg/toolchains
+ * make printf() and cprintf() use stdarg.h instead of rolling their own
+ * x86-64's calling conventions for var-args are more complex
+ * notably the first six arguments may be passed in registers
+ * implement 32bit-to-64bit kernel entry glue (entry64.S)
+ * qemu cannot load 64bit ELF kernels
+ * sounds like most linux bootloaders don't support that yet either
+ * implement a 32bit multiboot header and shim
+ * setup an initial identity-mapped and kernel-address-mapped
+ pagetable since you cannot enter 64bit mode without paging on
+
+IN PROGRESS
+ * introduce new type (currently 'uintp' for unsigned integer the size
+ of a pointer) to use in all the places where Xv6 assumes pointers and
+ integers are interchangeable.
+ * almost all warnings squashed
+ * consider a better name. addr_t?
+ * update various assembly stubs (x86.h) for 64bit mode (ldtr,lgdt,etc)
+ * implement 64bit mmu support (vm.c)
+
+TODO
+ * implement 64bit GDT and TSS setup
+ * implement 64bit interrupt support and IDT setup
+ * implement 64bit task switch glue
+ * adjust process entry code for 64bit
+ * adjust syscall assembly for 64bit
+ * verify usermode works
+ * verify usertests work
+ * update boot sector to support multiboot extended header
+ * the ELF header no longer handles the entrypoint
+ * test on real hardware
+ * get 32bit build working again
+
+THINGS TO FIX LATER / NICE TO HAVE
+ * I wish qemu had a "halt-on-exception" mode. Would help early bringup
+ * gdb pukes when qemu switches from 32bit to 64bit mode
+ * this made debugging the mode change entertaining
+ * for now attach gdb after the switch
+ * move userspace entry code to ulib
+
+LESSONS LEARNED / UNRESOLVED
+ * using 1GB pages in the initial page table did not work
+ * In qemu you can do cheesy debugging by writing bytes to port 0x3f8
+ (the first UART) without any normal UART setup.
+ * Xv6:32 creates kernel mappings (separate copies) in every process's
+ page table, roughly 70k pages worth, consuming about 280MB of ram
+ for page tables.
+
+IMPLEMENTATION NOTES
+ * use "kernel" addressing mode for kernel compilation
+ * kernel virtual address space 0xFFFFFFFF80000000:0xFFFFFFFFFF000000
+ * linear-map first 1GB of RAM to kernel virtual address space
+ * linear-map first 1GB of RAM 1:1 during boot
+
