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

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Message-ID: <68b075b8a9b26_22d98294dc@iweiny-mobl.notmuch>
Date: Thu, 28 Aug 2025 10:28:56 -0500
From: Ira Weiny <ira.weiny@...el.com>
To: Yan Zhao <yan.y.zhao@...el.com>, 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

Yan Zhao wrote:
> On Tue, Aug 26, 2025 at 05:05:19PM -0700, Sean Christopherson wrote:

[snip]

> > @@ -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.

I'm wondering if we are trying to over-architect this.

Should we even allow KVM_TDX_FINALIZE_VM to race with
KVM_TDX_INIT_MEM_REGION?  What is the use case for that?

It seems a basic sanity check/KVM_BUG_ON would suffice to tell the user;
Don't start adding memory dynamically until yall have finalized the VM.

Ira