KVM: x86: Unload MMUs during vCPU destruction, not before

KVM: x86: Unload MMUs during vCPU destruction, not before

When destroying a VM, unload a vCPU's MMUs as part of normal vCPU freeing,
instead of as a separate prepratory action.  Unloading MMUs ahead of time
is a holdover from commit 7b53aa565084 ("KVM: Fix vcpu freeing for guest
smp"), which "fixed" a rather egregious flaw where KVM would attempt to
free *all* MMU pages when destroying a vCPU.

At the time, KVM would spin on all MMU pages in a VM when free a single
vCPU, and so would hang due to the way KVM pins and zaps root pages
(roots are invalidated but not freed if they are pinned by a vCPU).

  static void free_mmu_pages(struct kvm_vcpu *vcpu)
  {
        struct kvm_mmu_page *page;

        while (!list_empty(&vcpu->kvm->active_mmu_pages)) {
                page = container_of(vcpu->kvm->active_mmu_pages.next,
                                    struct kvm_mmu_page, link);
                kvm_mmu_zap_page(vcpu->kvm, page);
        }
        free_page((unsigned long)vcpu->mmu.pae_root);
  }

Now that KVM doesn't try to free all MMU pages when destroying a single
vCPU, there's no need to unpin roots prior to destroying a vCPU.

Note!  While KVM mostly destroys all MMUs before calling
kvm_arch_destroy_vm() (see commit f00be0cae4e6 ("KVM: MMU: do not free
active mmu pages in free_mmu_pages()")), unpinning MMU roots during vCPU
destruction will unfortunately trigger remote TLB flushes, i.e. will try
to send requests to all vCPUs.

Happily, thanks to commit 27592ae8dbe4 ("KVM: Move wiping of the kvm->vcpus
array to common code"), that's a non-issue as freed vCPUs are naturally
skipped by xa_for_each_range(), i.e. by kvm_for_each_vcpu().  Prior to that
commit, KVM x86 rather stupidly freed vCPUs one-by-one, and _then_
nullified them, one-by-one.  I.e. triggering a VM-wide request would hit a
use-after-free.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250224235542.2562848-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>