Re: [PATCH] xen/arm: gic-v2: disable interrupt bypass on CPU shutdown

[Mykola Kvach <xakep.amatop@xxxxxxxxx>]

Hi Michal,

Thank you for the review.

On Mon, Apr 20, 2026 at 11:38 AM Orzel, Michal <michal.orzel@xxxxxxx> wrote:
>
> Hmm, this landed in my junk folder.
>
> On 10/04/2026 11:36, Mykola Kvach wrote:
> > From: Mykola Kvach <mykola_kvach@xxxxxxxx>
> >
> > The GICv2 CPU shutdown path currently writes 0 to GICC_CTLR.
> >
> > Per IHI0048B.b section 2.3.1, clearing IRQBypDisGrp{0,1} and
> > FIQBypDisGrp{0,1} selects bypass rather than deasserted interrupt
> > outputs when the CPU interface stops driving them. Tables 2-2 and 2-3
> > show that a zeroed GICC_CTLR can fall back to the legacy IRQ/FIQ inputs
> > instead of fully disabling the interface.
> >
> > Fix this by reading GICC_CTLR, setting the bypass-disable bits, and
> > clearing both group-enable bits before writing the value back. Keep the
> > existing GICC_CTL_ENABLE definition for the init path and use a separate
> > mask for the shutdown-side group-enable handling.
> IIUC we don't need to worry about not setting the bypass-disable bits in cpu
> init (we only set group 0 and EOI) because they are relevant only when the 
> bit 0
> is disabled i.e. the path this patch changes?

Yes, that is my understanding as well.

In cpu_init(), Xen enables the CPU interface by programming
GICC_CTL_ENABLE | GICC_CTL_EOI. In that state, the GIC CPU interface
drives the interrupt outputs, so leaving the bypass-disable bits clear
does not cause fallback to the legacy bypass path.

The issue is specific to cpu_disable(): once the group-enable bits are
cleared, leaving the bypass-disable bits clear can select the legacy
IRQ/FIQ bypass inputs instead of fully deasserting the outputs.

>
> >
> > Section 2.3.2 also states that wakeup event signals remain available
> > even when both GIC interrupt signaling and interrupt bypass are
> > disabled, so disabling bypass does not break the power-management use
> > case, i.e. suspend modes.
> >
> > Fixes: 5e40a1b4351e ("arm: SMP CPU shutdown")
> > Signed-off-by: Mykola Kvach <mykola_kvach@xxxxxxxx>
> Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>
>
> ~Michal
>

Best regards,
Mykola