[PATCH 2/7] KVM: x86/mmu: Return RET_PF* instead of 1 in kvm_mmu_page_fault()

[Yan Zhao]

Return RET_PF* (excluding RET_PF_EMULATE/RET_PF_CONTINUE/RET_PF_INVALID)
instead of 1 in kvm_mmu_page_fault().

The callers of kvm_mmu_page_fault() are KVM page fault handlers (i.e.,
npf_interception(), handle_ept_misconfig(), __vmx_handle_ept_violation(),
kvm_handle_page_fault()). They either check if the return value is > 0 (as
in npf_interception()) or pass it further to vcpu_run() to decide whether
to break out of the kernel loop and return to the user when r <= 0.
Therefore, returning any positive value is equivalent to returning 1.

Warn if r == RET_PF_CONTINUE (which should not be a valid value) to ensure
a positive return value.

This is a preparation to allow TDX's EPT violation handler to check the
RET_PF* value and retry internally for RET_PF_RETRY.

No functional changes are intended.

Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
---
 arch/x86/kvm/mmu/mmu.c          | 10 +++++++++-
 arch/x86/kvm/mmu/mmu_internal.h | 12 +++++++++---
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index eedc6ff37b89..53dcf600e934 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -6120,8 +6120,16 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
 	else if (r == RET_PF_SPURIOUS)
 		vcpu->stat.pf_spurious++;
 
+	/*
+	 * None of handle_mmio_page_fault(), kvm_mmu_do_page_fault(), or
+	 * kvm_mmu_write_protect_fault() return RET_PF_CONTINUE.
+	 * kvm_mmu_do_page_fault() only uses RET_PF_CONTINUE internally to
+	 * indicate continuing the page fault handling until to the final
+	 * page table mapping phase.
+	 */
+	WARN_ON_ONCE(r == RET_PF_CONTINUE);
 	if (r != RET_PF_EMULATE)
-		return 1;
+		return r;
 
 emulate:
 	return x86_emulate_instruction(vcpu, cr2_or_gpa, emulation_type, insn,
diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index 957636660149..4fde91cade1b 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -315,9 +315,7 @@ int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault);
  * tracepoints via TRACE_DEFINE_ENUM() in mmutrace.h
  *
  * Note, all values must be greater than or equal to zero so as not to encroach
- * on -errno return values.  Somewhat arbitrarily use '0' for CONTINUE, which
- * will allow for efficient machine code when checking for CONTINUE, e.g.
- * "TEST %rax, %rax, JNZ", as all "stop!" values are non-zero.
+ * on -errno return values.
  */
 enum {
 	RET_PF_CONTINUE = 0,
@@ -329,6 +327,14 @@ enum {
 	RET_PF_SPURIOUS,
 };
 
+/*
+ * Define RET_PF_CONTINUE as 0 to allow for
+ * - efficient machine code when checking for CONTINUE, e.g.
+ *   "TEST %rax, %rax, JNZ", as all "stop!" values are non-zero,
+ * - kvm_mmu_do_page_fault() to return other RET_PF_* as a positive value.
+ */
+static_assert(RET_PF_CONTINUE == 0);
+
 static inline void kvm_mmu_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
 						     struct kvm_page_fault *fault)
 {
-- 
2.43.2