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

[Julien Grall <julien.grall@xxxxxxx>]

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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