Re: [Xen-devel] [PATCH] xen: arm: load FDT below 0.5G

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 30 July 2013 10:57, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Tue, 2013-07-30 at 10:53 +0100, Julien Grall wrote:
>> On 29 July 2013 15:46, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
>> > On Wed, 2013-07-24 at 15:37 +0100, Ian Campbell wrote:
>> >> BTW I asked for confirmation on linux-arm-kernel but I don't see that in
>> >> my l-a-k folder or the archives -- did you get the copy I CCd to you?
>> >> Subject is "Requirements for FDT load address on ARM".
>> >
>> > There was some discussion on L-A-K and I think we know what to do, see
>> > http://thread.gmane.org/gmane.linux.ports.arm.kernel/254538 and
>> > http://thread.gmane.org/gmane.linux.ports.arm.kernel/255716
>> >
>> > The updated patch would look like the below (although I may try and
>> > combine place+check_overlap+load into a single function, which tries a
>> > bit harder to find a good address by looping until overlap == false).
>>
>> Do you plan to support position-dependent DOM0 kernel? If no, as the
>> kernel will be relocated to the same address, I don't think we need to
>> automatically find the DTB base address.
>
> What do you mean? Even if the kernel is not PIC it will still have to
> decompress itself somewhere and things move around in RAM etc and
> potentially clobber the DTB.

You can only guess theses different addresses. Xen is unable to know
the final memory layout for DOM 0.

If I understand the email from Nicolas Pitre
(http://thread.gmane.org/gmane.linux.ports.arm.kernel/254538/focus=254640)
we are safe in most of the case with the current computation of the
DTB address.

> But anyway, yes we should be supporting PIC kernel, in fact I have a
> suspicion we only support PIC kernels...
>
>> > commit 176dafbc2f916f369450e647a33806c13270be15
>> > Author: Ian Campbell <ian.campbell@xxxxxxxxxx>
>> > Date:   Tue Jul 23 06:05:29 2013 +0100
>> >
>> >     xen: arm: load FDT at correct address
>> >
>> >     The 64-bit Documentation/arm64/booting.txt requires that the FDT be 
>> > below
>> >     512MiB. On 32-bit things are less clear but discussion at
>> >     http://thread.gmane.org/gmane.linux.ports.arm.kernel/254538 suggests 
>> > that
>> >     below 256MiB should be good. I've submitted a patch to
>> >     Documentation/arm/Booting to cover this.
>> >
>> >     Also note that 64-bit requires 2MiB alignment for the FDT. We already 
>> > do this
>> >     but update the comment.
>> >
>> >     Locating the fdt needs to be called after prepare_kernel so we know 
>> > which sort
>> >     of kernel it is, so split out into a separate "place_fdt".
>> >
>> >     Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
>>        Acked-by: Julien Grall <julien.grall@xxxxxxxxxx>
>
> Thanks, I'm not totally sure this will be the final form but if it
> doesn't change too much I will keep this.
>
>



-- 
Julien Grall

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