KVM: arm64: Don't arm a hrtimer for an already pending timer

When fully emulating a timer, we back it with a hrtimer that is
armver on vcpu_load(). However, we do this even if the timer is
already pending.

This causes spurious interrupts to be taken, though the guest
doesn't observe them (the interrupt is already pending).

Although this is a waste of precious cycles, this isn't the
end of the world with the current state of KVM. However, this
can lead to a situation where a guest doesn't make forward
progress anymore with NV.

Fix it by checking that if the timer is already pending
before arming a new hrtimer. Also drop the hrtimer cancelling,
which is useless, by construction.

Reported-by: D Scott Phillips <scott@os.amperecomputing.com>
Fixes: bee038a ("KVM: arm/arm64: Rework the timer code to use a timer_map")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230112123829.458912-2-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/arch_timer.c

@@ -428,10 +428,8 @@ static void timer_emulate(struct arch_timer_context *ctx)
 	 * scheduled for the future.  If the timer cannot fire at all,
 	 * then we also don't need a soft timer.
 	 */
-	if (!kvm_timer_irq_can_fire(ctx)) {
-		soft_timer_cancel(&ctx->hrtimer);
+	if (should_fire || !kvm_timer_irq_can_fire(ctx))
 		return;
-	}
 
 	soft_timer_start(&ctx->hrtimer, kvm_timer_compute_delta(ctx));
 }