Re: [Xen-devel] [PATCH v2 34/35] xen/arm32: head: Setup HTTBR in enable_mmu() and add missing isb

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 7/30/19 10:26 PM, Stefano Stabellini wrote:
> On Mon, 22 Jul 2019, Julien Grall wrote:
> > At the moment, HTTBR is setup in create_page_tables(). This is fine as
> > it is called by every CPUs.
> >
> > However, such assumption may not hold in the future. To make change
> > easier, the HTTBR is not setup in enable_mmu().
> >
> > Take the opportunity to add the missing isb() to ensure the HTTBR is
> > seen before the MMU is turned on.
> >
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> >
> > ---
> >      Changes in v2:
> >          - Patch added
> > ---
> >   xen/arch/arm/arm32/head.S | 8 ++++++--
> >   1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
> > index 6d55a2119a..8a1e272aab 100644
> > --- a/xen/arch/arm/arm32/head.S
> > +++ b/xen/arch/arm/arm32/head.S
> > @@ -373,8 +373,6 @@ create_page_tables:
> >           /* Write Xen's PT's paddr into the HTTBR */
> This comment needs to be moved
Good spot!

> >           ldr   r4, =boot_pgtable
> >           add   r4, r4, r10            /* r4 := paddr (boot_pagetable) */
> > -        mov   r5, #0                 /* r4:r5 is paddr (boot_pagetable) */
> > -        mcrr  CP64(r4, r5, HTTBR)
> Interestingly r5 is not clobbered by create_page_tables anymore, we need
> to update the comment on top.
I knew documenting the clobbered registers are going to cause some 
trouble when updating the code :). I will fix it in the next version.
Cheers,

--
Julien Grall

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