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

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Message-ID: <aK7CvEcYXhr/p2wY@yzhao56-desk.sh.intel.com>
Date: Wed, 27 Aug 2025 16:33:00 +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 05/12] KVM: TDX: Drop superfluous page pinning in
 S-EPT management

On Tue, Aug 26, 2025 at 05:05:15PM -0700, 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().
Hmm, we implemented exactly the same patch at [1], where we explained the
potential problems of not holding page refcount, and the explored various
approaches, and related considerations.

[1] https://lore.kernel.org/all/20250807094241.4523-1-yan.y.zhao@intel.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
>