On Tue, Jan 8, 2013 at 9:55 AM, David Woodhouse <dw...@infradead.org> wrote:
> On Tue, 2013-01-08 at 09:41 +0000, David Woodhouse wrote:
>> Note that OVMF is already broken for the case of booting a 32-bit kernel
>> from 64-bit firmware, even for the legacy handover case which *ought* to
>> work. We need to switch back into 32-bit mode before jumping to the
>> 32-bit entry point. Rather than just jumping to the non-existent 64-bit
>> entry point in a 32-bit kernel. I haven't addressed that here.
>
> It turns out that it's fairly much impossible for the bootloader to tell
> whether the 64-bit entry point at 0x200 actually exists. In theory I've
> been told that you can look at the ELF payload of the bzImage payload
> and its ELF class and the entry point in its headers. But that would
> involve decompressing it first. With the compression method du jour of
> the kernel. As far as I can tell, the 64-bit entry point (at the
> undocumented offset 0x200) is completely unusable when booting a
> bzImage. Vivek?
>
> So let's *unconditionally* switch back to 32-bit mode and jump to the
> 32-bit entry point which *is* guaranteed to work, like grub does...
>
> Does this look sane? It would need translating for JumpToKernel.asm.
> And it's assuming that both kernel and bootparams are below 4GiB.
>
> diff -u b/edk2/OvmfPkg/Library/LoadLinuxLib/X64/JumpToKernel.S
> b/edk2/OvmfPkg/Library/LoadLinuxLib/X64/JumpToKernel.S
> --- b/edk2/OvmfPkg/Library/LoadLinuxLib/X64/JumpToKernel.S
> +++ b/edk2/OvmfPkg/Library/LoadLinuxLib/X64/JumpToKernel.S
> @@ -25,9 +25,39 @@
>
> #------------------------------------------------------------------------------
> ASM_PFX(JumpToKernel):
> movq %rdx, %rsi
> - addq $0x200, %rcx
> - callq %rcx
> - ret
> + movq %rcx, %rbx
> + movq $0x10, %rax
> + shl $32, %rax
> + orq $1f, %rax
> + pushq %rax
> + retf
> +1: // Now in compatibility mode
> +.code32
> + movl $0x18, %eax
> + movl %eax, %ds
> + movl %eax, %es
> + movl %eax, %fs
> + movl %eax, %gs
> + movl %eax, %ss
> +
> + // Disable paging
> + movl %cr0, %eax
> + btcl $31, %eax
> + movl %eax, %cr0
> +
> + // Disable long mode in EFER
> + movl $0x0c0000080, %ecx
> + rdmsr
> + btcl $8, %eax
> + wrmsr
> +
> + // Disable PAE
> + movl %cr0, %eax
> + btcl $5, %eax
> + movl %eax, %cr0
> +
> + jmp %ebx
> +.code64
Hopefully Matt can look this over.
Have fun with the MASM version of this, and I know I'll cringe when I
see the 32-bit code written in 64-bit assembly mode. :)
>
> #------------------------------------------------------------------------------
> # VOID
> --- a/edk2/OvmfPkg/Library/LoadLinuxLib/LinuxGdt.c
> +++ b/edk2/OvmfPkg/Library/LoadLinuxLib/LinuxGdt.c
> @@ -83,7 +83,11 @@ STATIC GDT_ENTRIES GdtTemplate = {
> 0x0, // base 0
> 0x0,
> 0x09A, // present, ring 0, data, expand-up, writable
> +#ifdef MDE_CPU_IA32
> 0x0CF, // page-granular, 32-bit
> +#else
> + 0x0AF, // compatibility mode
> +#endif
I assume the 64-bit EFI path will not use this entry?
-Jordan
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