linux-kernel - Re: [RFC PATCH 05/12] KVM: TDX: Drop superfluous page pinning in S-EPT management

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Message-ID: <aLAAX2fQFZh51ONY@yzhao56-desk.sh.intel.com>
Date: Thu, 28 Aug 2025 15:08:15 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Ira Weiny <ira.weiny@...el.com>
CC: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
	<pbonzini@...hat.com>, <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Michael Roth <michael.roth@....com>, Vishal Annapurve
	<vannapurve@...gle.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: Re: [RFC PATCH 05/12] KVM: TDX: Drop superfluous page pinning in
 S-EPT management

On Wed, Aug 27, 2025 at 07:36:46PM -0500, Ira Weiny wrote:
> Sean Christopherson wrote:
> > Don't explicitly pin pages when mapping pages into the S-EPT, guest_memfd
> > doesn't support page migration in any capacity, i.e. there are no migrate
> > callbacks because guest_memfd pages *can't* be migrated.  See the WARN in
> > kvm_gmem_migrate_folio().
> 
> I like the fact this removes a poorly named function tdx_unpin() as well.
> 
> That said, concerning gmem tracking page reference, I have some questions.
> In the TDX.PAGE.AUG path, [via kvm_gmem_get_pfn()] gmem takes a folio
kvm_mmu_finish_page_fault() will decrease the folio refcount.

> reference whereas the TDX.PAGE.ADD path [via kvm_gmem_populate()] does not
> take a folio reference.
> 
> Why are these paths different?
> 
> For this patch.
> 
> Reviewed-by: Ira Weiny <ira.weiny@...el.com>
> 
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/kvm/vmx/tdx.c | 28 ++++------------------------
> >  1 file changed, 4 insertions(+), 24 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > index 1724d82c8512..9fb6e5f02cc9 100644
> > --- a/arch/x86/kvm/vmx/tdx.c
> > +++ b/arch/x86/kvm/vmx/tdx.c
> > @@ -1586,29 +1586,22 @@ void tdx_load_mmu_pgd(struct kvm_vcpu *vcpu, hpa_t root_hpa, int pgd_level)
> >  	td_vmcs_write64(to_tdx(vcpu), SHARED_EPT_POINTER, root_hpa);
> >  }
> >  
> > -static void tdx_unpin(struct kvm *kvm, struct page *page)
> > -{
> > -	put_page(page);
> > -}
> > -
> >  static int tdx_mem_page_aug(struct kvm *kvm, gfn_t gfn,
> > -			    enum pg_level level, struct page *page)
> > +			    enum pg_level level, kvm_pfn_t pfn)
> >  {
> >  	int tdx_level = pg_level_to_tdx_sept_level(level);
> >  	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> > +	struct page *page = pfn_to_page(pfn);
> >  	gpa_t gpa = gfn_to_gpa(gfn);
> >  	u64 entry, level_state;
> >  	u64 err;
> >  
> >  	err = tdh_mem_page_aug(&kvm_tdx->td, gpa, tdx_level, page, &entry, &level_state);
> > -	if (unlikely(tdx_operand_busy(err))) {
> > -		tdx_unpin(kvm, page);
> > +	if (unlikely(tdx_operand_busy(err)))
> >  		return -EBUSY;
> > -	}
> >  
> >  	if (KVM_BUG_ON(err, kvm)) {
> >  		pr_tdx_error_2(TDH_MEM_PAGE_AUG, err, entry, level_state);
> > -		tdx_unpin(kvm, page);
> >  		return -EIO;
> >  	}
> >  
> > @@ -1642,29 +1635,18 @@ static int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn,
> >  				     enum pg_level level, kvm_pfn_t pfn)
> >  {
> >  	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> > -	struct page *page = pfn_to_page(pfn);
> >  
> >  	/* TODO: handle large pages. */
> >  	if (KVM_BUG_ON(level != PG_LEVEL_4K, kvm))
> >  		return -EINVAL;
> >  
> > -	/*
> > -	 * Because guest_memfd doesn't support page migration with
> > -	 * a_ops->migrate_folio (yet), no callback is triggered for KVM on page
> > -	 * migration.  Until guest_memfd supports page migration, prevent page
> > -	 * migration.
> > -	 * TODO: Once guest_memfd introduces callback on page migration,
> > -	 * implement it and remove get_page/put_page().
> > -	 */
> > -	get_page(page);
> > -
> >  	/*
> >  	 * Read 'pre_fault_allowed' before 'kvm_tdx->state'; see matching
> >  	 * barrier in tdx_td_finalize().
> >  	 */
> >  	smp_rmb();
> >  	if (likely(kvm_tdx->state == TD_STATE_RUNNABLE))
> > -		return tdx_mem_page_aug(kvm, gfn, level, page);
> > +		return tdx_mem_page_aug(kvm, gfn, level, pfn);
> >  
> >  	return tdx_mem_page_record_premap_cnt(kvm, gfn, level, pfn);
> >  }
> > @@ -1715,7 +1697,6 @@ static int tdx_sept_drop_private_spte(struct kvm *kvm, gfn_t gfn,
> >  		return -EIO;
> >  	}
> >  	tdx_clear_page(page);
> > -	tdx_unpin(kvm, page);
> >  	return 0;
> >  }
> >  
> > @@ -1795,7 +1776,6 @@ static int tdx_sept_zap_private_spte(struct kvm *kvm, gfn_t gfn,
> >  	if (tdx_is_sept_zap_err_due_to_premap(kvm_tdx, err, entry, level) &&
> >  	    !KVM_BUG_ON(!atomic64_read(&kvm_tdx->nr_premapped), kvm)) {
> >  		atomic64_dec(&kvm_tdx->nr_premapped);
> > -		tdx_unpin(kvm, page);
> >  		return 0;
> >  	}
> >  
> > -- 
> > 2.51.0.268.g9569e192d0-goog
> > 
> 
>