[U-Boot] ARMv8 linking: how to properly accommodate the exception vector table?

[Alexander Graf]

On 02/20/2019 12:19 PM, Andre Przywara wrote:
> Hi,
>
> I was looking for a solution to some linker problem and was hoping that
> some linker cracks here on the list have some idea:
>
> The generic ARMv8 ("arm64") code provides an exception vector table,
> which architecturally requires a 2KB alignment. At the moment we don't
> take any special measures about it, so we end up with a random padding
> between 0 and 2044 bytes before the table:
>
> 0 bytes                   n * 2KB
> |                            |
>   <code before table><padding>|<vector table> <code after table>
>
> The vector table takes up a bit more than 1KB, so we happily use the
> space after that and can't afford to just waste the rest of the 2KB slot
> (by putting it at the end of the code, for instance).
>
> Depending on the link order some code ends up before the table, other
> behind it, with seemingly random effects on the resulting code size, due
> to the padding.
>
> This is quite annoying with the Allwinner SPLs, which are quite tight.
> On the H6 it seems like most code additions lead to a build break at the
> moment due to this, which needs fixing. Actually it should affect all
> ARMv8 boards, but it might just not be critical for most of them.
>
>
> I was wondering if we can tell the linker in the (SPL) linker script to
> drop code into *two* possible regions, so that we can use a fixed
> location for the vector table somewhere in the middle, and tell the
> linker to fill up the space before *and* after that at will, without
> requiring us to specify the exact link order of code.
> I can *manually* arrange the code to make best use of the space, but
> this is very fragile and breaks every so often, and is thus not sustainable.
>
> Another idea would be to put the exception table at the end of the image
> (at least for sunxi), then fill the remaining <1KB with some data. But I
> didn't find a low hanging fruit data block which fills the gap nicely.
>
> If anyone has any ideas on how to overcome this, I would be very grateful
> to hear about it.

Can't you just put the exception table at 0x0 and make the first 
instruction be the "b reset" from start.S? Then (if we need that vector, 
I don't know) overwrite it with the real exception vector down the road.


Alex