linux-kernel - Re: [RFC PATCH 09/12] KVM: TDX: Fold tdx_mem_page_record_premap

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Message-ID: <aK7Ji3kAoDaEYn3h@yzhao56-desk.sh.intel.com>
Date: Wed, 27 Aug 2025 17:02:03 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Michael Roth <michael.roth@....com>, "Ira
 Weiny" <ira.weiny@...el.com>, Vishal Annapurve <vannapurve@...gle.com>, "Rick
 Edgecombe" <rick.p.edgecombe@...el.com>
Subject: Re: [RFC PATCH 09/12] KVM: TDX: Fold
 tdx_mem_page_record_premap_cnt() into its sole caller

On Tue, Aug 26, 2025 at 05:05:19PM -0700, Sean Christopherson wrote:
> Fold tdx_mem_page_record_premap_cnt() into tdx_sept_set_private_spte() as
> providing a one-off helper for effectively three lines of code is at best a
> wash, and splitting the code makes the comment for smp_rmb()  _extremely_
> confusing as the comment talks about reading kvm->arch.pre_fault_allowed
> before kvm_tdx->state, but the immediately visible code does the exact
> opposite.
> 
> Opportunistically rewrite the comments to more explicitly explain who is
> checking what, as well as _why_ the ordering matters.
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/vmx/tdx.c | 49 ++++++++++++++++++------------------------
>  1 file changed, 21 insertions(+), 28 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index b7559ea1e353..e4b70c0dbda3 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -1608,29 +1608,6 @@ static int tdx_mem_page_aug(struct kvm *kvm, gfn_t gfn,
>  	return 0;
>  }
>  
> -/*
> - * KVM_TDX_INIT_MEM_REGION calls kvm_gmem_populate() to map guest pages; the
> - * callback tdx_gmem_post_populate() then maps pages into private memory.
> - * through the a seamcall TDH.MEM.PAGE.ADD().  The SEAMCALL also requires the
> - * private EPT structures for the page to have been built before, which is
> - * done via kvm_tdp_map_page(). nr_premapped counts the number of pages that
> - * were added to the EPT structures but not added with TDH.MEM.PAGE.ADD().
> - * The counter has to be zero on KVM_TDX_FINALIZE_VM, to ensure that there
> - * are no half-initialized shared EPT pages.
> - */
> -static int tdx_mem_page_record_premap_cnt(struct kvm *kvm, gfn_t gfn,
> -					  enum pg_level level, kvm_pfn_t pfn)
> -{
> -	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> -
> -	if (KVM_BUG_ON(kvm->arch.pre_fault_allowed, kvm))
> -		return -EIO;
> -
> -	/* nr_premapped will be decreased when tdh_mem_page_add() is called. */
> -	atomic64_inc(&kvm_tdx->nr_premapped);
> -	return 0;
> -}
> -
>  static int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn,
>  				     enum pg_level level, kvm_pfn_t pfn)
>  {
> @@ -1641,14 +1618,30 @@ static int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn,
>  		return -EIO;
>  
>  	/*
> -	 * Read 'pre_fault_allowed' before 'kvm_tdx->state'; see matching
> -	 * barrier in tdx_td_finalize().
> +	 * Ensure pre_fault_allowed is read by kvm_arch_vcpu_pre_fault_memory()
> +	 * before kvm_tdx->state.  Userspace must not be allowed to pre-fault
> +	 * arbitrary memory until the initial memory image is finalized.  Pairs
> +	 * with the smp_wmb() in tdx_td_finalize().
>  	 */
>  	smp_rmb();
> -	if (likely(kvm_tdx->state == TD_STATE_RUNNABLE))
> -		return tdx_mem_page_aug(kvm, gfn, level, pfn);
>  
> -	return tdx_mem_page_record_premap_cnt(kvm, gfn, level, pfn);
> +	/*
> +	 * If the TD isn't finalized/runnable, then userspace is initializing
> +	 * the VM image via KVM_TDX_INIT_MEM_REGION.  Increment the number of
> +	 * pages that need to be initialized via TDH.MEM.PAGE.ADD (PAGE.ADD
> +	 * requires a pre-existing S-EPT mapping).  KVM_TDX_FINALIZE_VM checks
> +	 * the counter to ensure all mapped pages have been added to the image,
> +	 * to prevent running the TD with uninitialized memory.
To prevent the mismatch between mirror EPT and the S-EPT?

e.g., Before KVM_TDX_FINALIZE_VM,
if userspace performs a zap after the TDH.MEM.PAGE.ADD, the page will be removed
from the S-EPT. The count of nr_premapped will not change after the successful
TDH.MEM.RANGE.BLOCK and TDH.MEM.PAGE.REMOVE.

As a result, the TD will still run with uninitialized memory.

> +	 */
> +	if (unlikely(kvm_tdx->state != TD_STATE_RUNNABLE)) {
> +		if (KVM_BUG_ON(kvm->arch.pre_fault_allowed, kvm))
> +			return -EIO;
> +
> +		atomic64_inc(&kvm_tdx->nr_premapped);
> +		return 0;
> +	}
> +
> +	return tdx_mem_page_aug(kvm, gfn, level, pfn);
>  }
>  
>  static int tdx_sept_drop_private_spte(struct kvm *kvm, gfn_t gfn,
> -- 
> 2.51.0.268.g9569e192d0-goog
>