Re: [PATCH -v6 0/3] x86 boot: 32-bit boot protocol

From: H. Peter Anvin
Date: Mon Oct 22 2007 - 14:42:49 EST


Huang, Ying wrote:
This patchset defines a 32-bit boot protocol for x86 platform,
adds an extensible boot parameter passing mechanism, export the boot
parameters via sysfs.

The patchset has been tested against kernel of git version
v2.6.23-6623-g55b70a0 on x86_64 and i386.


Hi Huang,

This patchset should be rebased on top of Rusty's changes; the rebase is fairly trivial and I was originally intending to simply commit the rebase as-is, with the boot protocol version bumped to 2.08.

However, the documentation section is simply wrong in a number of places. In particular:

+In 32-bit boot protocol, the first step in loading a Linux kernel
+should still be to load the real-mode code and then examine the kernel
+header at offset 0x01f1. But, it is not necessary to load all
+real-mode code, just first 4K bytes traditionally known as "zero page"
+is needed.

This is incorrect. The zeropage (which really is better referred to as struct boot_param) should be initialized to all zero, except for the setup header (starting at offset 0x1f0 or 0x1f1(*)) to the length specified either by boot protocol version or by the byte at offset 0x201.

+At entry, the CPU must be in 32-bit protected mode with paging
+disabled; the CS and DS must be 4G flat segments; %esi holds the base
+address of the "zero page"; %esp, %ebp, %edi should be zero.

You also need to have a GDT loaded with the selectors for __BOOT_CS (0x10) and __BOOT_DS (0x18) containing appropriate values, and you should enter with interrupts disabled. For safety, set up ES and SS as well as DS.

The bit about %esp, %ebp and %edi being zero is nonsense, although specifying at least %ebp == %edi == 0 for future use isn't a bad idea. On the other hand, %ebx *is* supposed to be zero.

The documentation in zero-page.txt is wrong when it comes to protocol versions. Most of these fields are ancient, and only a handful of the remainder can be tied to specific protocol versions.

+ struct setup_data {
+ u64 next;
+ u32 type;
+ u32 len;
+ u8 data[0];
+ } __attribute__((packed));

Why packed?

Time permitting, I might rewrite this myself, but it may be quicker for you to update it.

-hpa

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