KVM: mmu: allow page tables to be in read-only slots

Page tables in a read-only memory slot will currently cause a triple
fault because the page walker uses gfn_to_hva and it fails on such a slot.

OVMF uses such a page table; however, real hardware seems to be fine with
that as long as the accessed/dirty bits are set.  Save whether the slot
is readonly, and later check it when updating the accessed and dirty bits.

Reviewed-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 9 insertions, 5 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index bf040c4e02b3..979bff485fb0 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1058,11 +1058,15 @@ unsigned long gfn_to_hva(struct kvm *kvm, gfn_t gfn)
 EXPORT_SYMBOL_GPL(gfn_to_hva);
 
 /*
- * The hva returned by this function is only allowed to be read.
- * It should pair with kvm_read_hva() or kvm_read_hva_atomic().
+ * If writable is set to false, the hva returned by this function is only
+ * allowed to be read.
  */
-static unsigned long gfn_to_hva_read(struct kvm *kvm, gfn_t gfn)
+unsigned long gfn_to_hva_prot(struct kvm *kvm, gfn_t gfn, bool *writable)
 {
+        struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
+        if (writable)
+                *writable = !memslot_is_readonly(slot);
+
         return __gfn_to_hva_many(gfn_to_memslot(kvm, gfn), gfn, NULL, false);
 }
 
@@ -1430,7 +1434,7 @@ int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset,
         int r;
         unsigned long addr;
 
-        addr = gfn_to_hva_read(kvm, gfn);
+        addr = gfn_to_hva_prot(kvm, gfn, NULL);
         if (kvm_is_error_hva(addr))
                 return -EFAULT;
         r = kvm_read_hva(data, (void __user *)addr + offset, len);
@@ -1468,7 +1472,7 @@ int kvm_read_guest_atomic(struct kvm *kvm, gpa_t gpa, void *data,
         gfn_t gfn = gpa >> PAGE_SHIFT;
         int offset = offset_in_page(gpa);
 
-        addr = gfn_to_hva_read(kvm, gfn);
+        addr = gfn_to_hva_prot(kvm, gfn, NULL);
         if (kvm_is_error_hva(addr))
                 return -EFAULT;
         pagefault_disable();