[PATCH 3/4] arm64: fix hyp mode mismatch detection

[Mark Rutland]

On Fri, Mar 13, 2015 at 08:21:19PM +0000, Ard Biesheuvel wrote:
> On 13 March 2015 at 17:14, Mark Rutland <mark.rutland at arm.com> wrote:
> > Commit 828e9834e9a5b7e6 ("arm64: head: create a new function for setting
> > the boot_cpu_mode flag") added BOOT_CPU_MODE_EL1, a nonzero value
> > replacing uses of zero. However it failed to update __boot_cpu_mode
> > appropriately.
> >
> > A CPU booted at EL2 writes BOOT_CPU_MODE_EL2 to __boot_cpu_mode[0], and
> > a CPU booted at EL1 writes BOOT_CPU_MODE_EL1 to __boot_cpu_mode[1].
> > Later is_hyp_mode_mismatched() determines there to be a mismatch if
> > __boot_cpu_mode[0] != __boot_cpu_mode[1].
> >
> > If all CPUs are booted at EL1, __boot_cpu_mode[0] will be set to
> > BOOT_CPU_MODE_EL1, but __boot_cpu_mode[1] will retain its initial value
> > of zero, and is_hyp_mode_mismatched will erroneously determine that the
> > boot modes are mismatched. This hasn't been a problem so far, but later
> > patches which will make use of is_hyp_mode_mismatched() expect it to
> > work correctly.
> >
> > This patch initialises __boot_cpu_mode[1] to BOOT_CPU_MODE_EL1, fixing
> > the erroneous mismatch detection when all CPUs are booted at EL1.
> >
> 
> Maybe it's just me, but isn't it *much* easier to understand to
> initialise both values to 0, and use 'both are non-zero' as the error
> condition?
> 'HYP mode available' would then be '__boot_cpu_mode[0] ==
> BOOT_CPU_MODE_EL2 && __boot_cpu_mode[1] == 0'

I agree that change would make this easier to follow.

Marc, are you happy with Ard's proposed change?

Mark.

> > Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> > Cc: Catalin Marinas <catalin.marinas at arm.com>
> > Cc: Marc Zyngier <marc.zyngier at arm.com>
> > Cc: Will Deacon <will.deacon at arm.com>
> > ---
> >  arch/arm64/kernel/head.S | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> > index 07f9305..d17649d 100644
> > --- a/arch/arm64/kernel/head.S
> > +++ b/arch/arm64/kernel/head.S
> > @@ -588,7 +588,7 @@ ENDPROC(set_cpu_boot_mode_flag)
> >         .align  L1_CACHE_SHIFT
> >  ENTRY(__boot_cpu_mode)
> >         .long   BOOT_CPU_MODE_EL2
> > -       .long   0
> > +       .long   BOOT_CPU_MODE_EL1
> >         .popsection
> >
> >  #ifdef CONFIG_SMP
> > --
> > 1.9.1
> >
>