Re: [PATCH v3 23/23] arm64: Panic when VHE and non VHE CPUs coexist

Catalin Marinas
2016-02-09T12:02:17Z

On Mon, Feb 08, 2016 at 04:24:03PM +0000, Mark Rutland wrote:
> On Mon, Feb 08, 2016 at 04:04:46PM +0000, Catalin Marinas wrote:
> > On Wed, Feb 03, 2016 at 06:00:16PM +0000, Marc Zyngier wrote:
> > > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> > > index 6f2f377..f9b6a5b 100644
> > > --- a/arch/arm64/kernel/head.S
> > > +++ b/arch/arm64/kernel/head.S
> > > @@ -578,7 +578,24 @@ ENTRY(set_cpu_boot_mode_flag)
> > > 1: str w20, [x1] // This CPU has booted in EL1
> > > dmb sy
> > > dc ivac, x1 // Invalidate potentially stale cache line
> > > + adr_l x1, __run_cpu_mode
> > > + ldr w0, [x1]
> > > + mrs x20, CurrentEL
> > > + cbz x0, skip_el_check
> > > + cmp x0, x20
> > > + bne mismatched_el
> > > ret
> > > +skip_el_check: // Only the first CPU gets to set the rule
> > > + str w20, [x1]
> > > + dmb sy
> > > + dc ivac, x1 // Invalidate potentially stale cache line
> > > + ret
> > > +mismatched_el:
> > > + str w20, [x1, #4]
> > > + dmb sy
> > > + dc ivac, x1 // Invalidate potentially stale cache line
> > > +1: wfi
> > > + b 1b
> > > ENDPROC(set_cpu_boot_mode_flag)
> >
> > Do we need to wait for the D-cache maintenance completion before
> > entering WFI (like issuing a DSB)? Same for the skip_el_check path
> > before the RET.
>
> We would need that to complete the maintenance, yes.
>
> However, given we're going into WFI immediately afterwards, and not
> signalling completion to other CPUs, what we gain is somewhat
> questionable.
>
> Perhaps it's always better to do the maintenance on the read side (for
> consistency with [1]).
>
> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-February/404661.html

While [1] looks clearer to me, it seems that it is not consistent with
the rest of the head.S file. Maybe we can clean them up as a separate
patch and leave the dc ivac here. We also seem to miss barriers in other
similar situations in head.S.

--
Catalin