linux-kernel - Re: [PATCH 14/21] KVM: TDX: Implement hooks to propagate changes of TDP MMU mirror page table

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Message-ID: <128db3b3-f971-497c-910c-b6e2f9bafaf6@redhat.com>
Date: Tue, 10 Sep 2024 11:33:34 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
 "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai" <kai.huang@...el.com>
Cc: "nik.borisov@...e.com" <nik.borisov@...e.com>,
 "dmatlack@...gle.com" <dmatlack@...gle.com>,
 "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
 "Zhao, Yan Y" <yan.y.zhao@...el.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 14/21] KVM: TDX: Implement hooks to propagate changes of
 TDP MMU mirror page table

On 9/9/24 23:03, Edgecombe, Rick P wrote:
> KVM code tends to be less wrapped and a tdx_vm_destory would be a
> oneline function. So I think it fits in other ways.

Yes, no problem there.  Sometimes one-line functions are ok (see 
ept_sync_global() case elsewhere in the series), sometimes they're 
overkill, especially if they wrap a function defined in the same file as 
the wrapper.

>>> +        * which causes gmem invalidation to zap all spte.
>>> +        * Population is only allowed after KVM_TDX_INIT_VM.
>>> +        */
>> What does the second sentence ("Population ...")  meaning?  Why is it
>> relevant here?
>>
> How about:
> /*
>   * HKID is released after all private pages have been removed,
>   * and set before any might be populated. Warn if zapping is
>   * attempted when there can't be anything populated in the private
>   * EPT.
>   */
> 
> But actually, I wonder if we need to remove the KVM_BUG_ON(). I think if you did
> a KVM_PRE_FAULT_MEMORY and then deleted the memslot you could hit it?

I think all paths to handle_removed_pt() are safe:

__tdp_mmu_zap_root
         tdp_mmu_zap_root
                 kvm_tdp_mmu_zap_all
                         kvm_arch_flush_shadow_all
                                 kvm_flush_shadow_all
                                         kvm_destroy_vm (*)
                                         kvm_mmu_notifier_release (*)
                 kvm_tdp_mmu_zap_invalidated_roots
                         kvm_mmu_zap_all_fast (**)
kvm_tdp_mmu_zap_sp
         kvm_recover_nx_huge_pages (***)


(*) only called at destroy time
(**) only invalidates direct roots
(***) shouldn't apply to TDX I hope?

Paolo