Re: [PATCH early-RFC 4/5] xen/arm: mm: Rework switch_ttbr()

[Julien Grall <julien@xxxxxxx>]

On 25/03/2022 13:47, Bertrand Marquis wrote:
> Hi Julien,
Hi Bertrand,

> > On 9 Mar 2022, at 12:20, Julien Grall <julien@xxxxxxx> wrote:
> >
> > From: Julien Grall <jgrall@xxxxxxxxxx>
> >
> > At the moment, switch_ttbr() is switching the TTBR whilst the MMU is
> > still on.
> >
> > Switching TTBR is like replacing existing mappings with new ones. So
> > we need to follow the break-before-make sequence.
> >
> > In this case, it means the MMU needs to be switched off while the
> > TTBR is updated. In order to disable the MMU, we need to first
> > jump to an identity mapping.
> >
> > Rename switch_ttbr() to switch_ttbr_id() and create an helper on
> > top to temporary map the identity mapping and call switch_ttbr()
> > via the identity address.
> >
> > switch_ttbr_id() is now reworked to temporarily turn off the MMU
> > before updating the TTBR.
> >
> > We also need to make sure the helper switch_ttbr() is part of the
> > identity mapping. So move _end_boot past it.
> >
> > Take the opportunity to instruction cache flush as the operation is
> > only necessary when the memory is updated.
> Your code is actually remove the instruction cache invalidation so
> this sentence is a bit misleading.
I forgot to add the word "remove" in the sentence.

> Also an open question: shouldn’t we flush the data cache ?
Do you mean clean/invalidate to PoC/PoU? Something else?

> As we switch from one TTBR to an other, there might be some data
> in the cache dependent that could be flushed while the MMU is off 
I am a bit confused. Those flush could also happen with the MMU on. So 
how turning off the MMU would result to a problem? Note that the data 
cache is still enabled during the switch.
> or
> that would have no mapping once it is reactivated.
The cache line will be flushed at some point in the future. I would 
argue if the caller need it earlier, then it should make sure to issue 
the flush before switch_ttbr().
Cheers,

--
Julien Grall