KVM: arm64: timers: Convert per-vcpu virtual offset to a global value

Having a per-vcpu virtual offset is a pain. It needs to be synchronized
on each update, and expands badly to a setup where different timers can
have different offsets, or have composite offsets (as with NV).

So let's start by replacing the use of the CNTVOFF_EL2 shadow register
(which we want to reclaim for NV anyway), and make the virtual timer
carry a pointer to a VM-wide offset.

This simplifies the code significantly. It also addresses two terrible bugs:

- The use of CNTVOFF_EL2 leads to some nice offset corruption
  when the sysreg gets reset, as reported by Joey.

- The kvm mutex is taken from a vcpu ioctl, which goes against
  the locking rules...

Reported-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230224173915.GA17407@e124191.cambridge.arm.com
Tested-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20230224191640.3396734-1-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

include/kvm/arm_arch_timer.h

@@ -23,6 +23,19 @@ enum kvm_arch_timer_regs {
 	TIMER_REG_CTL,
 };
 
+struct arch_timer_offset {
+	/*
+	 * If set, pointer to one of the offsets in the kvm's offset
+	 * structure. If NULL, assume a zero offset.
+	 */
+	u64	*vm_offset;
+};
+
+struct arch_timer_vm_data {
+	/* Offset applied to the virtual timer/counter */
+	u64	voffset;
+};
+
 struct arch_timer_context {
 	struct kvm_vcpu			*vcpu;
 
@@ -32,6 +45,8 @@ struct arch_timer_context {
 	/* Emulated Timer (may be unused) */
 	struct hrtimer			hrtimer;
 
+	/* Offset for this counter/timer */
+	struct arch_timer_offset	offset;
 	/*
 	 * We have multiple paths which can save/restore the timer state onto
 	 * the hardware, so we need some way of keeping track of where the