On 03/02/16 11:50, Borislav Petkov wrote:
> On Wed, Mar 02, 2016 at 10:39:05AM -0800, H. Peter Anvin wrote:
>> Well, we definitely should use %rip-relative addressing if we can.
>
> Right you are.
>
>> However, even so I believe this breaks if the kernel is loaded anywhere
>> but its default load address. I think we need to do something like:
>>
>> movq stack_start(%rip), %rax
>> leaq __START_KERNEL_map(%rip), %rdx
>> subq %rdx, %rax
>> movq %rax, %rsp
>>
>> The use of temporary registers avoids clobbering a valid stack pointer
>> for even a single instruction if we are given one.
>
> Yeah, we should be prudent and make this as sturdy as possible. I did this:
>
> CONFIG_PHYSICAL_START=0x100beef
>
> and it aligned startup_64 up to ffffffff82000000. It seems to boot fine
> in kvm. But better safe than sorry.
>
You're not actually testing anything as the real issue is what happens
with a relocating bootloader. That's okay; I think we can be pretty
sure the above works by inspection.
-hpa
