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, 19 insertions, 1 deletions

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 043330159179..ad75d77999d0 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -99,6 +99,7 @@ struct guest_walker {
         pt_element_t prefetch_ptes[PTE_PREFETCH_NUM];
         gpa_t pte_gpa[PT_MAX_FULL_LEVELS];
         pt_element_t __user *ptep_user[PT_MAX_FULL_LEVELS];
+        bool pte_writable[PT_MAX_FULL_LEVELS];
         unsigned pt_access;
         unsigned pte_access;
         gfn_t gfn;
@@ -235,6 +236,22 @@ static int FNAME(update_accessed_dirty_bits)(struct kvm_vcpu *vcpu,
                 if (pte == orig_pte)
                         continue;
 
+                /*
+                 * If the slot is read-only, simply do not process the accessed
+                 * and dirty bits.  This is the correct thing to do if the slot
+                 * is ROM, and page tables in read-as-ROM/write-as-MMIO slots
+                 * are only supported if the accessed and dirty bits are already
+                 * set in the ROM (so that MMIO writes are never needed).
+                 *
+                 * Note that NPT does not allow this at all and faults, since
+                 * it always wants nested page table entries for the guest
+                 * page tables to be writable.  And EPT works but will simply
+                 * overwrite the read-only memory to set the accessed and dirty
+                 * bits.
+                 */
+                if (unlikely(!walker->pte_writable[level - 1]))
+                        continue;
+
                 ret = FNAME(cmpxchg_gpte)(vcpu, mmu, ptep_user, index, orig_pte, pte);
                 if (ret)
                         return ret;
@@ -309,7 +326,8 @@ retry_walk:
                         goto error;
                 real_gfn = gpa_to_gfn(real_gfn);
 
-                host_addr = gfn_to_hva(vcpu->kvm, real_gfn);
+                host_addr = gfn_to_hva_prot(vcpu->kvm, real_gfn,
+                                            &walker->pte_writable[walker->level - 1]);
                 if (unlikely(kvm_is_error_hva(host_addr)))
                         goto error;