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

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: 6e01b7601dfe ("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>