KVM: x86/mmu: Check and free obsolete roots in kvm_mmu_reload()

Check request KVM_REQ_MMU_FREE_OBSOLETE_ROOTS to free obsolete roots in
kvm_mmu_reload() to prevent kvm_mmu_reload() from seeing a stale obsolete
root.

Since kvm_mmu_reload() can be called outside the
vcpu_enter_guest() path (e.g., kvm_arch_vcpu_pre_fault_memory()), it may be
invoked after a root has been marked obsolete and before vcpu_enter_guest()
is invoked to process KVM_REQ_MMU_FREE_OBSOLETE_ROOTS and set root.hpa to
invalid. This causes kvm_mmu_reload() to fail to load a new root, which
can lead to kvm_arch_vcpu_pre_fault_memory() being stuck in the while
loop in kvm_tdp_map_page() since RET_PF_RETRY is always returned due to
is_page_fault_stale().

Keep the existing check of KVM_REQ_MMU_FREE_OBSOLETE_ROOTS in
vcpu_enter_guest() since the cost of kvm_check_request() is negligible,
especially a check that's guarded by kvm_request_pending().

Export symbol of kvm_mmu_free_obsolete_roots() as kvm_mmu_reload() is
inline and may be called outside of kvm.ko.

Fixes: 6e01b76 ("KVM: x86: Implement kvm_arch_vcpu_pre_fault_memory()")
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://lore.kernel.org/r/20250318013333.5817-1-yan.y.zhao@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: yanzhao56

arch/x86/kvm/mmu.h

@@ -104,6 +104,9 @@ void kvm_mmu_track_write(struct kvm_vcpu *vcpu, gpa_t gpa, const u8 *new,
 
 static inline int kvm_mmu_reload(struct kvm_vcpu *vcpu)
 {
+	if (kvm_check_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu))
+		kvm_mmu_free_obsolete_roots(vcpu);
+
 	/*
 	 * Checking root.hpa is sufficient even when KVM has mirror root.
 	 * We can have either:

arch/x86/kvm/mmu/mmu.c

@@ -5974,6 +5974,7 @@ void kvm_mmu_free_obsolete_roots(struct kvm_vcpu *vcpu)
 	__kvm_mmu_free_obsolete_roots(vcpu->kvm, &vcpu->arch.root_mmu);
 	__kvm_mmu_free_obsolete_roots(vcpu->kvm, &vcpu->arch.guest_mmu);
 }
+EXPORT_SYMBOL_GPL(kvm_mmu_free_obsolete_roots);
 
 static u64 mmu_pte_write_fetch_gpte(struct kvm_vcpu *vcpu, gpa_t *gpa,
 				    int *bytes)