[PATCH] irqchip/gic-v3: Fix priority comparison when non-secure priorities are used

[Marc Zyngier]

On Fri, 20 Aug 2021 14:31:39 +0100,
Alexandru Elisei <alexandru.elisei at arm.com> wrote:
> 
> Hello,
> 
> Pending Marc's testing (I realized I don't have any hardware to test
> this on at the moment), this patch looks correct to me. One comment
> below.

Seems good so far. I tested it both in a VM, on a FIQ==1 host, and on
D05, which runs with FIQ==0. Can't be more broken than it was... ;-)

> 
> On 8/11/21 6:15 PM, Chen-Yu Tsai wrote:
> > When non-secure priorities are used, compared to the raw priority set,
> > the value read back from RPR is also right-shifted by one and the
> > highest bit set.
> >
> > Add a macro to do the modifications to the raw priority when doing the
> > comparison against the RPR value. This corrects the pseudo-NMI behavior
> > when non-secure priorities in the GIC are used. Tested on 5.10 with
> > the "IPI as pseudo-NMI" series [1] applied on MT8195.
> >
> > [1] https://lore.kernel.org/linux-arm-kernel/1604317487-14543-1-git-send-email-sumit.garg@linaro.org/
> >
> > Fixes: 336780590990 ("irqchip/gic-v3: Support pseudo-NMIs when SCR_EL3.FIQ == 0")
> > Signed-off-by: Chen-Yu Tsai <wenst at chromium.org>
> > ---
> >  drivers/irqchip/irq-gic-v3.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> > index e0f4debe64e1..e7a0b55413db 100644
> > --- a/drivers/irqchip/irq-gic-v3.c
> > +++ b/drivers/irqchip/irq-gic-v3.c
> > @@ -100,6 +100,15 @@ EXPORT_SYMBOL(gic_pmr_sync);
> >  DEFINE_STATIC_KEY_FALSE(gic_nonsecure_priorities);
> >  EXPORT_SYMBOL(gic_nonsecure_priorities);
> >  
> > +#define GICD_INT_RPR_PRI(priority)					\
> > +	({								\
> > +		u32 __priority = (priority);				\
> > +		if (static_branch_unlikely(&gic_nonsecure_priorities))	\
> > +			__priority = 0x80 | (__priority >> 1);		\
> > +									\
> > +		__priority;						\
> > +	})
> 
> Would you mind adding a comment to the macro explaining why it's
> needed? This behaviour is rather subtle and I'm hoping it will save
> someone's time at some point in the future. I'm thinking something
> like this (please ignore it if you can think of something better):
> 
> When the Non-secure world has access to group 0 interrupts
> (SCR_EL3.FIQ = 0), reading the ICC_RPR_EL1 register will return the
> Distributor's view of the interrupt priority. When GIC security is
> enabled (GICD_CTLR.DS = 0), the interrupt priority written by
> software is moved to the Non-secure range by the Distributor.  If
> both are true (which is the situation where gic_nonsecure_priorities
> gets enabled), then we need to shift down the priority programmed by
> software if we want match it against the value returned from
> ICC_RPR_EL1.
> 
> With a comment added:
> 
> Reviewed-by: Alexandru Elisei <alexandru.elisei at arm.com>

Let me fold this into the commit and push it out again.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.