Re: [RFC PATCH v5 20/45] KVM: x86/mmu: Allocate/free S-EPT pages using tdx_{alloc,free}_control_page()

[Huang, Kai]

> 
> Option #2 would be to immediately free the page in tdx_sept_reclaim_private_sp(),
> so that pages that freed via handle_removed_pt() don't defer freeing the S-EPT
> page table (which, IIUC, is safe since the TDX-Module forces TLB flushes and exits).
> 
> I really, really don't like this option (if it even works).
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index ae7b9beb3249..4726011ad624 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -2014,7 +2014,15 @@ static void tdx_sept_reclaim_private_sp(struct kvm *kvm, gfn_t gfn,
>          */
>         if (KVM_BUG_ON(is_hkid_assigned(to_kvm_tdx(kvm)), kvm) ||
>             tdx_reclaim_page(virt_to_page(sp->external_spt)))
> -               sp->external_spt = NULL;
> +               goto out;
> +
> +       /*
> +        * Immediately free the control page, as the TDX subsystem doesn't
> +        * support freeing pages from RCU callbacks.
> +        */
> +       tdx_free_control_page((unsigned long)sp->external_spt);
> +out:
> +       sp->external_spt = NULL;
>  }
>  
>  void tdx_deliver_interrupt(struct kvm_lapic *apic, int delivery_mode,

I don't think this is so bad, given we already have a bunch of

	is_mirror_sp(sp)
		kvm_x86_call(xx_external_spt)(..);

in TDP MMU code?

I suppose this won't make a lot of difference, but does below make you
slightly happier?

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 3181406c5e0b..3588265098a8 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -55,8 +55,7 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
 
 static void tdp_mmu_free_sp(struct kvm_mmu_page *sp)
 {
-       if (sp->external_spt)
-               kvm_x86_call(free_external_sp)((unsigned long)sp-
>external_spt);
+       WARN_ON_ONCE(sp->external_spt);
        free_page((unsigned long)sp->spt);
        kmem_cache_free(mmu_page_header_cache, sp);
 }
@@ -457,8 +456,17 @@ static void handle_removed_pt(struct kvm *kvm,
tdp_ptep_t pt, bool shared)
                                    old_spte, FROZEN_SPTE, level, shared);
        }
 
-       if (is_mirror_sp(sp))
+       if (is_mirror_sp(sp)) {
                kvm_x86_call(reclaim_external_sp)(kvm, base_gfn, sp);
+               /*
+                * Immediately free the control page, as the TDX subsystem
doesn't
+                * support freeing pages from RCU callbacks.
+                */
+               if (sp->external_spt) {
+                       kvm_x86_call(free_external_sp)((unsigned long)sp-
>external_spt);
+                       sp->external_spt = NULL;
+               }
+       }
 
        call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);
 }