[PATCH] KVM: x86/mmu: Rephrase comment about synthetic PFERR flags in #PF handler

[Sean Christopherson <seanjc@google.com>]

Reword the BUILD_BUG_ON() comment in the legacy #PF handler to explicitly
describe how asserting that synthetic PFERR flags are limited to bits 31:0
protects KVM against inadvertently passing a synthetic flag to the common
page fault handler.

No functional change intended.

Suggested-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/mmu.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 8d7115230739..2421d971ce1b 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4599,7 +4599,10 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
 	if (WARN_ON_ONCE(error_code >> 32))
 		error_code = lower_32_bits(error_code);
 
-	/* Ensure the above sanity check also covers KVM-defined flags. */
+	/*
+	 * Restrict KVM-defined flags to bits 63:32 so that it's impossible for
+	 * them to conflict with #PF error codes, which are limited to 32 bits.
+	 */
 	BUILD_BUG_ON(lower_32_bits(PFERR_SYNTHETIC_MASK));
 
 	vcpu->arch.l1tf_flush_l1d = true;

base-commit: b9adc10edd4e14e66db4f7289a88fdbfa45ae7a8
-- 
2.45.2.505.gda0bf45e8d-goog