KVM: MMU: Store nx bit for large page shadows

We need to distinguish between large page shadows which have the nx bit set
and those which don't. The problem shows up when booting a newer smp Linux
kernel, where the trampoline page (which is in real mode, which uses the
same shadow pages as large pages) is using the same mapping as a kernel data
page, which is mapped using nx, causing kvm to spin on that page.

Signed-off-by: Avi Kivity <[e-mail]>

drivers/kvm/kvm.h

···
  *   bits 4:7 - page table level for this shadow (1-4)
  *   bits 8:9 - page table quadrant for 2-level guests
  *   bit   16 - "metaphysical" - gfn is not a real page (huge page/real mode)
- *   bits 17:18 - "access" - the user and writable bits of a huge page pde
+ *   bits 17:19 - "access" - the user, writable, and nx bits of a huge page pde
  */
 union kvm_mmu_page_role {
 	unsigned word;
···
 		unsigned quadrant : 2;
 		unsigned pad_for_nice_hex_output : 6;
 		unsigned metaphysical : 1;
-		unsigned hugepage_access : 2;
+		unsigned hugepage_access : 3;
 	};
 };
 

drivers/kvm/paging_tmpl.h

···
 			metaphysical = 1;
 			hugepage_access = *guest_ent;
 			hugepage_access &= PT_USER_MASK | PT_WRITABLE_MASK;
+			if (*guest_ent & PT64_NX_MASK)
+				hugepage_access |= (1 << 2);
 			hugepage_access >>= PT_WRITABLE_SHIFT;
 			table_gfn = (*guest_ent & PT_BASE_ADDR_MASK)
 				>> PAGE_SHIFT;