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

Yan Zhao
2025-01-13T02:12:30Z

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@intel.com>
---
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