irqchip/gic-v3: Force propagation of the active state with a read-back

JIRA: https://issues.redhat.com/browse/RHEL-62922

commit 464cb98
Author: Marc Zyngier <maz@kernel.org>
Date: Wed, 6 Nov 2024 08:44:18 +0000

    Christoffer reports that on some implementations, writing to
    GICR_ISACTIVER0 (and similar GICD registers) can race badly with a guest
    issuing a deactivation of that interrupt via the system register interface.

    There are multiple reasons to this:

     - this uses an early write-acknoledgement memory type (nGnRE), meaning
       that the write may only have made it as far as some interconnect
       by the time the store is considered "done"

     - the GIC itself is allowed to buffer the write until it decides to
       take it into account (as long as it is in finite time)

    The effects are that the activation may not have taken effect by the time
    the kernel enters the guest, forcing an immediate exit, or that a guest
    deactivation occurs before the interrupt is active, doing nothing.

    In order to guarantee that the write to the ISACTIVER register has taken
    effect, read back from it, forcing the interconnect to propagate the write,
    and the GIC to process the write before returning the read.

    Reported-by: Christoffer Dall <christoffer.dall@arm.com>
    Signed-off-by: Marc Zyngier <maz@kernel.org>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Acked-by: Christoffer Dall <christoffer.dall@arm.com>
    Cc: stable@vger.kernel.org
    Link: https://lore.kernel.org/all/20241106084418.3794612-1-maz@kernel.org

Signed-off-by: Charles Mirabile <cmirabil@redhat.com>

Author: Charles Mirabile

drivers/irqchip/irq-gic-v3.c

@@ -539,6 +539,13 @@ static int gic_irq_set_irqchip_state(struct irq_data *d,
 	}
 
 	gic_poke_irq(d, reg);
+
+	/*
+	 * Force read-back to guarantee that the active state has taken
+	 * effect, and won't race with a guest-driven deactivation.
+	 */
+	if (reg == GICD_ISACTIVER)
+		gic_peek_irq(d, reg);
 	return 0;
 }