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

[Wei Chen <Wei.Chen@xxxxxxx>]

Hi Julien,

> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: 2022年11月14日 5:43
> 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
> 
> 
> 
> 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.
> 

Yes, now it depends on the objects' order in Makefile. This should be
a risk.

> 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.
> 

Yes, we might place this new section after text.header.

Cheers,
Wei Chen

> Cheers,
> 
> --
> Julien Grall