Re: [PATCH 05/12] KVM: SEV: cleanup locking for KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM

[Peter Gonda <pgonda@google.com>]

On Mon, Nov 22, 2021 at 5:50 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> Encapsulate the handling of the migration_in_progress flag for both VMs in
> two functions sev_lock_two_vms and sev_unlock_two_vms.  It does not matter
> if KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM locks the source struct kvm a bit
> earlier, and this change 1) keeps the cleanup chain of labels smaller 2)
> makes it possible for KVM_CAP_VM_COPY_ENC_CONTEXT_FROM to reuse the logic.
>
> Cc: Peter Gonda <pgonda@google.com>
> Cc: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Reviewed-by: Peter Gonda <pgonda@google.com>

> ---
>  arch/x86/kvm/svm/sev.c | 50 ++++++++++++++++++++----------------------
>  1 file changed, 24 insertions(+), 26 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 75955beb3770..23a4877d7bdf 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -1543,28 +1543,37 @@ static bool is_cmd_allowed_from_mirror(u32 cmd_id)
>         return false;
>  }
>
> -static int sev_lock_for_migration(struct kvm *kvm)
> +static int sev_lock_two_vms(struct kvm *dst_kvm, struct kvm *src_kvm)
>  {
> -       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> +       struct kvm_sev_info *dst_sev = &to_kvm_svm(dst_kvm)->sev_info;
> +       struct kvm_sev_info *src_sev = &to_kvm_svm(src_kvm)->sev_info;
>
>         /*
> -        * Bail if this VM is already involved in a migration to avoid deadlock
> -        * between two VMs trying to migrate to/from each other.
> +        * Bail if these VMs are already involved in a migration to avoid
> +        * deadlock between two VMs trying to migrate to/from each other.

Nit: This comment will be a little stale once we use this function for
clone too. Is there a generic term we could use for migration and
clone?

>          */
> -       if (atomic_cmpxchg_acquire(&sev->migration_in_progress, 0, 1))
> +       if (atomic_cmpxchg_acquire(&dst_sev->migration_in_progress, 0, 1))

Nit: I guess the same comment can be made for |migration_in_progress|.

>                 return -EBUSY;
>
> -       mutex_lock(&kvm->lock);
> +       if (atomic_cmpxchg_acquire(&src_sev->migration_in_progress, 0, 1)) {
> +               atomic_set_release(&dst_sev->migration_in_progress, 0);
> +               return -EBUSY;
> +       }
>
> +       mutex_lock(&dst_kvm->lock);
> +       mutex_lock(&src_kvm->lock);
>         return 0;
>  }
>
> -static void sev_unlock_after_migration(struct kvm *kvm)
> +static void sev_unlock_two_vms(struct kvm *dst_kvm, struct kvm *src_kvm)
>  {
> -       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> +       struct kvm_sev_info *dst_sev = &to_kvm_svm(dst_kvm)->sev_info;
> +       struct kvm_sev_info *src_sev = &to_kvm_svm(src_kvm)->sev_info;
>
> -       mutex_unlock(&kvm->lock);
> -       atomic_set_release(&sev->migration_in_progress, 0);
> +       mutex_unlock(&dst_kvm->lock);
> +       mutex_unlock(&src_kvm->lock);
> +       atomic_set_release(&dst_sev->migration_in_progress, 0);
> +       atomic_set_release(&src_sev->migration_in_progress, 0);
>  }
>
>
> @@ -1665,15 +1674,6 @@ int svm_vm_migrate_from(struct kvm *kvm, unsigned int source_fd)
>         bool charged = false;
>         int ret;
>
> -       ret = sev_lock_for_migration(kvm);
> -       if (ret)
> -               return ret;
> -
> -       if (sev_guest(kvm)) {
> -               ret = -EINVAL;
> -               goto out_unlock;
> -       }
> -
>         source_kvm_file = fget(source_fd);
>         if (!file_is_kvm(source_kvm_file)) {
>                 ret = -EBADF;
> @@ -1681,13 +1681,13 @@ int svm_vm_migrate_from(struct kvm *kvm, unsigned int source_fd)
>         }
>
>         source_kvm = source_kvm_file->private_data;
> -       ret = sev_lock_for_migration(source_kvm);
> +       ret = sev_lock_two_vms(kvm, source_kvm);
>         if (ret)
>                 goto out_fput;
>
> -       if (!sev_guest(source_kvm)) {
> +       if (sev_guest(kvm) || !sev_guest(source_kvm)) {
>                 ret = -EINVAL;
> -               goto out_source;
> +               goto out_unlock;
>         }
>
>         src_sev = &to_kvm_svm(source_kvm)->sev_info;
> @@ -1727,13 +1727,11 @@ int svm_vm_migrate_from(struct kvm *kvm, unsigned int source_fd)
>                 sev_misc_cg_uncharge(cg_cleanup_sev);
>         put_misc_cg(cg_cleanup_sev->misc_cg);
>         cg_cleanup_sev->misc_cg = NULL;
> -out_source:
> -       sev_unlock_after_migration(source_kvm);
> +out_unlock:
> +       sev_unlock_two_vms(kvm, source_kvm);
>  out_fput:
>         if (source_kvm_file)
>                 fput(source_kvm_file);
> -out_unlock:
> -       sev_unlock_after_migration(kvm);
>         return ret;
>  }
>
> --
> 2.27.0
>
>