[RFC PATCH v2 13/23] KVM: x86: Introduce hugepage_set_guest_inhibit()

[Yan Zhao <yan.y.zhao@intel.com>]

TDX requires guests to accept S-EPT mappings created by the host KVM. Due
to the current implementation of the TDX module, if a guest accepts a GFN
at a lower level after KVM maps it at a higher level, the TDX module will
emulate an EPT violation VMExit to KVM instead of returning a size mismatch
error to the guest. If KVM fails to perform page splitting in the VMExit
handler, the guest's accept operation will be triggered again upon
re-entering the guest, causing a repeated EPT violation VMExit.

To facilitate passing the guest's accept level information to the KVM MMU
core and to prevent the repeated mapping of a GFN at different levels due
to different accept levels specified by different vCPUs, introduce the
interface hugepage_set_guest_inhibit(). This interface specifies across
vCPUs that mapping at a certain level is inhibited from the guest.

The KVM_LPAGE_GUEST_INHIBIT_FLAG bit is currently modified in one
direction (set), so no clear interface is provided.

Link: https://lore.kernel.org/all/a6ffe23fb97e64109f512fa43e9f6405236ed40a.camel@intel.com/ [1]
Suggested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
---
RFC v2:
- new in RFC v2
---
 arch/x86/kvm/mmu.h     |  3 +++
 arch/x86/kvm/mmu/mmu.c | 21 ++++++++++++++++++---
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index b122255c7d4e..c2d8819f3438 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -326,4 +326,7 @@ static inline bool kvm_is_gfn_alias(struct kvm *kvm, gfn_t gfn)
 {
 	return gfn & kvm_gfn_direct_bits(kvm);
 }
+
+void hugepage_set_guest_inhibit(struct kvm_memory_slot *slot, gfn_t gfn, int level);
+bool hugepage_test_guest_inhibit(struct kvm_memory_slot *slot, gfn_t gfn, int level);
 #endif
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 13910ae05f76..1c639286aac2 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -721,12 +721,14 @@ static struct kvm_lpage_info *lpage_info_slot(gfn_t gfn,
 }
 
 /*
- * The most significant bit in disallow_lpage tracks whether or not memory
- * attributes are mixed, i.e. not identical for all gfns at the current level.
+ * The most 2 significant bits in disallow_lpage tracks whether or not memory
+ * attributes are mixed, i.e. not identical for all gfns at the current level,
+ * or whether or not guest inhibits the current level of hugepage at the gfn.
  * The lower order bits are used to refcount other cases where a hugepage is
  * disallowed, e.g. if KVM has shadow a page table at the gfn.
  */
 #define KVM_LPAGE_MIXED_FLAG	BIT(31)
+#define KVM_LPAGE_GUEST_INHIBIT_FLAG   BIT(30)
 
 static void update_gfn_disallow_lpage_count(const struct kvm_memory_slot *slot,
 					    gfn_t gfn, int count)
@@ -739,7 +741,8 @@ static void update_gfn_disallow_lpage_count(const struct kvm_memory_slot *slot,
 
 		old = linfo->disallow_lpage;
 		linfo->disallow_lpage += count;
-		WARN_ON_ONCE((old ^ linfo->disallow_lpage) & KVM_LPAGE_MIXED_FLAG);
+		WARN_ON_ONCE((old ^ linfo->disallow_lpage) &
+			     (KVM_LPAGE_MIXED_FLAG | KVM_LPAGE_GUEST_INHIBIT_FLAG));
 	}
 }
 
@@ -1647,6 +1650,18 @@ static bool __kvm_rmap_zap_gfn_range(struct kvm *kvm,
 				 start, end - 1, can_yield, true, flush);
 }
 
+bool hugepage_test_guest_inhibit(struct kvm_memory_slot *slot, gfn_t gfn, int level)
+{
+	return lpage_info_slot(gfn, slot, level)->disallow_lpage & KVM_LPAGE_GUEST_INHIBIT_FLAG;
+}
+EXPORT_SYMBOL_GPL(hugepage_test_guest_inhibit);
+
+void hugepage_set_guest_inhibit(struct kvm_memory_slot *slot, gfn_t gfn, int level)
+{
+	lpage_info_slot(gfn, slot, level)->disallow_lpage |= KVM_LPAGE_GUEST_INHIBIT_FLAG;
+}
+EXPORT_SYMBOL_GPL(hugepage_set_guest_inhibit);
+
 /*
  * Split large leafs crossing the boundary of the specified range
  *
-- 
2.43.2