arm64: KVM: Add ARCH_WORKAROUND_2 support for guests

In order to offer ARCH_WORKAROUND_2 support to guests, we need
a bit of infrastructure.

Let's add a flag indicating whether or not the guest uses
SSBD mitigation. Depending on the state of this flag, allow
KVM to disable ARCH_WORKAROUND_2 before entering the guest,
and enable it when exiting it.

Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/kvm_host.h

@@ -216,6 +216,9 @@ struct kvm_vcpu_arch {
 	/* Exception Information */
 	struct kvm_vcpu_fault_info fault;
 
+	/* State of various workarounds, see kvm_asm.h for bit assignment */
+	u64 workaround_flags;
+
 	/* Guest debug state */
 	u64 debug_flags;