KVM: arm64: Restore missing ISB on nVHE __tlb_switch_to_guest

Commit a0e50aa3f4 ("KVM: arm64: Factor out stage 2 page table
data from struct kvm") dropped the ISB after __load_guest_stage2(),
only leaving the one that is required when the speculative AT
workaround is in effect.

As Andrew points it: "This alternative is 'backwards' to avoid a
double ISB as there is one in __load_guest_stage2 when the workaround
is active."

Restore the missing ISB, conditionned on the AT workaround not being
active.

Fixes: a0e50aa3f4 ("KVM: arm64: Factor out stage 2 page table data from struct kvm")
Reported-by: Andrew Scull <ascull@google.com>
Reported-by: Thomas Tai <thomas.tai@oracle.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/hyp/nvhe/tlb.c

@@ -31,7 +31,14 @@ static void __tlb_switch_to_guest(struct kvm_s2_mmu *mmu,
 		isb();
 	}
 
+	/*
+	 * __load_guest_stage2() includes an ISB only when the AT
+	 * workaround is applied. Take care of the opposite condition,
+	 * ensuring that we always have an ISB, but not two ISBs back
+	 * to back.
+	 */
 	__load_guest_stage2(mmu);
+	asm(ALTERNATIVE("isb", "nop", ARM64_WORKAROUND_SPECULATIVE_AT));
 }
 
 static void __tlb_switch_to_host(struct tlb_inv_context *cxt)