Re: [RFC v2 03/12] xen/arm32: head: Introduce an helper to flush the TLBs

[Julien Grall <julien@xxxxxxx>]

On 25/10/2022 10:53, Michal Orzel wrote:
> Hi Julien,
Hi Michal,

> On 22/10/2022 17:04, Julien Grall wrote:
> > From: Julien Grall <jgrall@xxxxxxxxxx>
> >
> > The sequence for flushing the TLBs is 4 instruction long and often
> > require an explanation how it works.
> s/require/requires/
Will fix it.

> > So create an helper and use it in the boot code (switch_ttbr() is left
> > alone for now).
> >
> > Note that in secondary_switched, we were also flushing the instruction
> > cache and branch predictor. Neither of them was necessary because:
> >      * We are only supporting IVIPT cache on arm32, so the instruction
> >        cache flush is only necessary when executable code is modified.
> >        None of the boot code is doing that.
> >      * The instruction cache is not invalidated and misprediction is not
> >        a problem at boot.
> >
> > Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> > ---
> >   xen/arch/arm/arm32/head.S | 31 ++++++++++++++++++-------------
> >   1 file changed, 18 insertions(+), 13 deletions(-)
> >
> > diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
> > index 163bd6596dec..aeaa8d105aeb 100644
> > --- a/xen/arch/arm/arm32/head.S
> > +++ b/xen/arch/arm/arm32/head.S
> > @@ -66,6 +66,21 @@
> >           add   \rb, \rb, r10
> >   .endm
> >
> > +/*
> > + * Flush local TLBs
> > + *
> > + * tmp1:    Scratch register
> I would love to adhere to the way of describing macro params like you did in 
> mov_w. This would mean:
> @tmp: scratch register
ok.

> Apart from that, the change looks ok.
>
> Question on the side:
> Why do we use nshst in assembly and ishst in TLB helper macro? > Is it because the latter is also used to flush the inner TLBs whereas 
the former only local ones?

nshst is sufficient for local TLBs flush.

The *_local helpers could also use 'nshst' but when they were introduced 
it wasn't clear whether 'nshst' was enough. I only got the confirmation 
after and never took the opportunity to update the code.
Cheers,

--
Julien Grall