Re: [Xen-devel] [PATCH] libxc: arm: Load the zImage to the rambase address + 2MB

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2014-06-30 at 15:05 +0100, Julien Grall wrote:
> Currently libxc is loading the kernel zImage at rambase + 32KB (0x8000).
> 
> Kernel are usually using 1MB (or 2MB) mapping for the early page table. If
> the kernel doesn't relocate itself this would require to virtual address
> starting at 0xXXXX8000. This is not part of the zImage protocol, but a
> convenience for Linux after the decompressor stage. Linux is able to
> load at any address in the memory and it will relocate itself to respect
> it own constraint.

Yes, because the boot protocol does not guarantee 2MB alignment, so if
the OS wants to rely on that then it is obliged to take care of
relocating itself.

> Load the zImage at rambase address + 2MB to make life easier for other
> kernel (such as FreeBSD, Mini-OS).

Whether or not it is easier these OSes *must* be prepared to be loaded
at any 4k aligned address. It sounds to me like you are hoping that
these OSes can *rely* on 2MB alignment, which is not the case. If they
make that assumption then they are buggy, regardless of whether it
happens to currently work with some loader or not.

> FWIW, this is already the case for DOM0.

On that basis I'm more inclined to change dom0 to use 32kb alignment.

Ian.


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