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

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

Hi,

On 17/12/2018 15:55, Andrii Anisov wrote:
> 
> 
> On 17.12.18 15:38, Julien Grall wrote:
>>>> 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.
> 
> I suppose you get me wrong.
> I'm saying that 128MB alignment requirement for RAM start might be relaxed by 
> kernel itself, if it has set ZRELADDR. But we, from XEN side, can not rely on 
> that, nor detect that.
> So we must follow RAM start alignment for compressed Linux kernel images (32 bit).

I didn't get you wrong. We are saying the same things :).

> 
>>>> 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.
> 
> It is rather a recommendation. A good way to choose the place for initramfs and 
> dtb. Because decompressor is pretty dumb and limited by 128MB addresses range.
> And on XEN side we follow this recommendation whenever it is possible.
> 
>> So technically, we should allow more than 128MB for the first bank
> 
> Although, it is not said that DTB or initramfs should reside in the first RAM 
> bank ;)

The documentation suggest to load the DTB and initramfs just above 128MB. I am 
not entirely how the kernel would behave if you put it in a separate bank far after.

Similarly, some version on Linux (i.e prior to 4.2) requires the DTB to within 
512MB from the kernel.

> 
>> 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.
> 
> The algorithm is the same, but it is buggy right now for smaller Dom0 sizes. The 
> lines
> 
>      const unsigned int min_low_order =
>          get_order_from_bytes(min_t(paddr_t, dom0_mem, MB(128)));
> 
> in `allocate_memory_11()` prone to end up with a wrong RAM start alignment.

Patches are welcomed.

Cheers,

-- 
Julien Grall

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