KVM: PPC: Disable NX for old magic page using guests

Old guests try to use the magic page, but map their trampoline code inside
of an NX region.

Since we can't fix those old kernels, try to detect whether the guest is sane
or not. If not, just disable NX functionality in KVM so that old guests at
least work at all. For newer guests, add a bit that we can set to keep NX
functionality available.

Signed-off-by: Alexander Graf <agraf@suse.de>

arch/powerpc/include/asm/kvm_host.h

@@ -631,6 +631,7 @@ struct kvm_vcpu_arch {
 #endif
 	unsigned long magic_page_pa; /* phys addr to map the magic page to */
 	unsigned long magic_page_ea; /* effect. addr to map the magic page to */
+	bool disable_kernel_nx;
 
 	int irq_type;		/* one of KVM_IRQ_* */
 	int irq_cpu_id;