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

[Julien Grall <julien@xxxxxxx>]

On 09/11/2022 07:36, Wei Chen wrote:
> Hi Julien,
Hi Wei,

> > -----Original Message-----
> > From: Julien Grall <julien@xxxxxxx>
> > Sent: 2022年11月7日 4:06
> > To: Wei Chen <Wei.Chen@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> > Cc: nd <nd@xxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>; Bertrand
> > Marquis <Bertrand.Marquis@xxxxxxx>; Volodymyr Babchuk
> > <Volodymyr_Babchuk@xxxxxxxx>; Henry Wang <Henry.Wang@xxxxxxx>
> > Subject: Re: [PATCH v6 08/11] xen/arm64: move MMU related code from head.S
> > to head_mmu.S
> >
> > 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.
> Quote from next email
> "
> 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.
> "
>
> OK. We will create a patch to check the size, and place it in the
> head of the series.
I thought a bit more about what I wrote. Xen binary should always start 
with the Image/Zimage header. At the moment, this is guaranteed because 
there is only one object using the section .text.header.
With the change introduced in this patch, there will be multiple objects 
using with the sections .text.header. This means we are relying on the 
compiler to always put the content of head.S first.
This is basically reverting to the behavior before commit 4267a33b19d 
("xen/build: put image header into a separate section").
Therefore we do need a separate section to be used for head_*.S and 
maybe part of head.S. This new section could be called text.idmap so we 
know what the section is used for.
Cheers,

--
Julien Grall