Re: [PATCH v6 08/11] xen/arm64: move MMU related code from head.S to head_mmu.S

[Julien Grall <julien@xxxxxxx>]

On 06/11/2022 20:06, Julien Grall wrote:
> Hi Wei,
>
> On 04/11/2022 10:07, Wei Chen wrote:
> > There are lots of MMU specific code in head.S. This code will not
> > be used in MPU systems. If we use #ifdef to gate them, the code
> > will become messy and hard to maintain. So we move MMU related
> > code to head_mmu.S, and keep common code still in head.S.
> I am afraid that you can't simply move the MMU code out of head.S 
> because this will break Xen when running using the identity map.
> This is because we only map the first 4KB of Xen with PA == VA. At the 
> moment, we guarantee it by having everything that needs to be used in 
> the identity mapping before _end_boot and checking at link time if this 
> fits in 4KB.
> Now that you moved the MMU code outside of head.S. We need to find a 
> different way to guarantee it. One way to do it would be to create a 
> section that would be used for everything that needs to be identity mapped.
Looking at the code this morning, I noticed that we already have the 
section ".text.header". For now, that should do the job. So we just need
to check the size of .text.header.

Ideally, checking the size should be done in a separate pre-patch so it 
is easier to review.
Cheers,

--
Julien Grall