Re: [PATCH v2 07/17] KVM: x86/mmu: Check PDPTRs before allocating PAE roots

[Paolo Bonzini]

On 08/04/21 17:48, Sean Christopherson wrote:
Freaking PDPTRs.  I was really hoping we could keep the lock and 
pages_available()
logic outside of the helpers.  What if kvm_mmu_load() reads the PDPTRs and
passes them into mmu_alloc_shadow_roots()?  Or is that too ugly?

The patch I have posted (though untested) tries to do that in a slightly 
less ugly way by pushing make_mmu_pages_available down to mmu_alloc_*_roots.

Paolo

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index efb41f31e80a..e3c4938cd665 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3275,11 +3275,11 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
         return 0;
  }

-static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
+static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu, u64 pdptrs[4])
  {
         struct kvm_mmu *mmu = vcpu->arch.mmu;
-       u64 pdptrs[4], pm_mask;
         gfn_t root_gfn, root_pgd;
+       u64 pm_mask;
         hpa_t root;
         int i;

@@ -3291,11 +3291,8 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)

         if (mmu->root_level == PT32E_ROOT_LEVEL) {
                 for (i = 0; i < 4; ++i) {
-                       pdptrs[i] = mmu->get_pdptr(vcpu, i);
-                       if (!(pdptrs[i] & PT_PRESENT_MASK))
-                               continue;
-
-                       if (mmu_check_root(vcpu, pdptrs[i] >> PAGE_SHIFT))
+                       if ((pdptrs[i] & PT_PRESENT_MASK) &&
+                           mmu_check_root(vcpu, pdptrs[i] >> PAGE_SHIFT))
                                 return 1;
                 }
         }
@@ -4844,21 +4841,33 @@ EXPORT_SYMBOL_GPL(kvm_mmu_reset_context);

  int kvm_mmu_load(struct kvm_vcpu *vcpu)
  {
-       int r;
+       struct kvm_mmu *mmu = vcpu->arch.mmu;
+       u64 pdptrs[4];
+       int r, i;

-       r = mmu_topup_memory_caches(vcpu, !vcpu->arch.mmu->direct_map);
+       r = mmu_topup_memory_caches(vcpu, !mmu->direct_map);
         if (r)
                 goto out;
         r = mmu_alloc_special_roots(vcpu);
         if (r)
                 goto out;
+
+       /*
+        * On SVM, reading PDPTRs might access guest memory, which might fault
+        * and thus might sleep.  Grab the PDPTRs before acquiring mmu_lock.
+        */
+       if (!mmu->direct_map && mmu->root_level == PT32E_ROOT_LEVEL) {
+               for (i = 0; i < 4; ++i)
+                       pdptrs[i] = mmu->get_pdptr(vcpu, i);
+       }
+
         write_lock(&vcpu->kvm->mmu_lock);
         if (make_mmu_pages_available(vcpu))
                 r = -ENOSPC;
         else if (vcpu->arch.mmu->direct_map)
                 r = mmu_alloc_direct_roots(vcpu);
         else
-               r = mmu_alloc_shadow_roots(vcpu);
+               r = mmu_alloc_shadow_roots(vcpu, pdptrs);
         write_unlock(&vcpu->kvm->mmu_lock);
         if (r)
                 goto out;