KVM: PPC: Align pt_regs in kvm_vcpu_arch structure

The H_ENTER_NESTED hypercall receives as second parameter the address
of a region of memory containing the values for the nested guest
privileged registers. We currently use the pt_regs structure contained
within kvm_vcpu_arch for that end.

Most hypercalls that receive a memory address expect that region to
not cross a 4K page boundary. We would want H_ENTER_NESTED to follow
the same pattern so this patch ensures the pt_regs structure sits
within a page.

Note: the pt_regs structure is currently 384 bytes in size, so
aligning to 512 is sufficient to ensure it will not cross a 4K page
and avoids punching too big a hole in struct kvm_vcpu_arch.
Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Murilo Opsfelder Araújo <muriloo@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220624142712.790491-1-farosas@linux.ibm.com

arch/powerpc/include/asm/kvm_host.h

@@ -523,7 +523,11 @@ struct kvm_vcpu_arch {
 	struct kvmppc_book3s_shadow_vcpu *shadow_vcpu;
 #endif
 
-	struct pt_regs regs;
+	/*
+	 * This is passed along to the HV via H_ENTER_NESTED. Align to
+	 * prevent it crossing a real 4K page.
+	 */
+	struct pt_regs regs __aligned(512);
 
 	struct thread_fp_state fp;