Re: [PATCH for-4.12 v2 2/2] xen/arm: Stop relocating Xen

[Julien Grall <julien.grall@arm.com>]

Hi,

On 17/12/2018 13:14, Andrii Anisov wrote:
> 
> 
> On 14.12.18 19:48, Julien Grall wrote:
>> Please try to reference the documentation (or code if lack of documentation) 
>> when making such statement.
> 
> Ah, yes, sure.
> 
>> AFAICT, Linux 32-bit [1] imposes the kernel to be loaded in the first 128MB of 
>> RAM. Nothing about the 128MB aligned RAM.
> 
> Right, the documentation sets a restriction for a compressed image to be loaded 
> in the first 128MB of RAM. But the comment in a decompressor code (and the code 
> itself) explicitly refer alignment [11].
> Calculating a RAM start address with this mask [22] give us a wrong value if the 
> real physical address is 64MB aligned, not 128MB. Then crash on decompressing. 
> You know, I stepped into that with J6 (arm32) while setting up thin Dom0 with 
> only 64MB RAM.

Thank you for the pointer.

> 
>> Linux 64-bit [1] requires to be loaded at a 2MB aligned address.
> 
> Great, 64-bit Linux documentation directly refers the alignment :)
> 
>> So technically allocating the RAM using a 2MB alignment should be enough.
> 
> For 64-bit and, maybe, raw 32-bit Linux kernel images.
> For 32-bit compressed Linux kernel - still 128MB aligned first bank is required.
> It can be changed on kernel side by setting ZRELADDR, but we can't designate 
> that from XEN runtime.

I also don't think this would be a good approach as we still want to keep the 
kernel as much as possible position independent.

> 
>> Yet we need to make sure the first bank is at least 128MB.
> 
> Well, I'm not sure the ARM64 documentation [33] or implementation mention the 
> size of the first bank. Except it should be enough to hold the kernel image [44].
> Also I would not treat [55] as a strict requirement to have 128MB in the first 
> bank. But we can stick at that to make things easier.

The size of the first bank is implicit on arm32. If you look at the 
Documentation/arm32/Booting.txt, the DTB/initramfs should be loaded just above 
128MB to avoid the decompressor to avoid overwrite them. So technically, we 
should allow more than 128MB for the first bank.

At the moment, the algorithm to load 64-bit and 32-bit Image are the same. Hence 
why I said we need at least 128MB in the first bank. I am open for using 
different algorithm if Dom0 should be smaller.

Cheers,

-- 
Julien Grall

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