Re: [PATCH 1/7] KVM: x86: Free vCPUs before freeing VM state

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 25, 2025, Yan Zhao wrote:
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 58b82d6fd77c..045c61cc7e54 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -12890,11 +12890,11 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
> >  		mutex_unlock(&kvm->slots_lock);
> >  	}
> >  	kvm_unload_vcpu_mmus(kvm);
> > +	kvm_destroy_vcpus(kvm);
> >  	kvm_x86_call(vm_destroy)(kvm);
> >  	kvm_free_msr_filter(srcu_dereference_check(kvm->arch.msr_filter, &kvm->srcu, 1));
> >  	kvm_pic_destroy(kvm);
> >  	kvm_ioapic_destroy(kvm);
> > -	kvm_destroy_vcpus(kvm);
> >  	kvfree(rcu_dereference_check(kvm->arch.apic_map, 1));
> >  	kfree(srcu_dereference_check(kvm->arch.pmu_event_filter, &kvm->srcu, 1));
> >  	kvm_mmu_uninit_vm(kvm);
> After this change, now the sequence is that
> 
> 1. kvm_arch_pre_destroy_vm()
> 2. kvm_arch_destroy_vm()
>    2.1 kvm_destroy_vcpus()
>    2.2 .vm_destroy hook
>    2.3 kvm_mmu_uninit_vm() --> mirror root ref is 1 upon here. Zap the mirror
>                                root and reclaim SETP page table pages.
>    2.4 .vm_free hook
> 
> Since TDX needs to reclaim the TDR page after reclaiming all other pages, we
> currently added a vm_free hook at 2.4, after 2.3.
> 
> Could we move kvm_mmu_uninit_vm() before the .vm_destroy hook and after
> kvm_destroy_vcpus()?
> 
> Or move the .vm_destroy hook after kvm_mmu_uninit_vm(), e.g. after
> kvm_page_track_cleanup()?

I would go for the first option.  I'll tack on a patch since I need to test all
of these flows anyways, and I would much prefer to change course sooner rather
than later if it doesn't work for whatever reason.

Is this comment accurate?

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1e5f6f820c0b..f5685f153e08 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12874,13 +12874,19 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
                mutex_unlock(&kvm->slots_lock);
        }
        kvm_destroy_vcpus(kvm);
+
+       /*
+        * Do final MMU teardown prior to calling into vendor code.  All pages
+        * that were donated to the TDX module, e.g. for S-EPT tables, need to
+        * be reclaimed before the VM metadata page can be freed.
+        */
+       kvm_mmu_uninit_vm(kvm);
        kvm_x86_call(vm_destroy)(kvm);
        kvm_free_msr_filter(srcu_dereference_check(kvm->arch.msr_filter, &kvm->srcu, 1));
        kvm_pic_destroy(kvm);
        kvm_ioapic_destroy(kvm);
        kvfree(rcu_dereference_check(kvm->arch.apic_map, 1));
        kfree(srcu_dereference_check(kvm->arch.pmu_event_filter, &kvm->srcu, 1));
-       kvm_mmu_uninit_vm(kvm);
        kvm_page_track_cleanup(kvm);
        kvm_xen_destroy_vm(kvm);
        kvm_hv_destroy_vm(kvm);