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

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Message-ID: <68b07bbc6d5de_22d982946f@iweiny-mobl.notmuch>
Date: Thu, 28 Aug 2025 10:54:36 -0500
From: Ira Weiny <ira.weiny@...el.com>
To: Yan Zhao <yan.y.zhao@...el.com>, 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

Yan Zhao wrote:
> 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.

Thanks,
Ira

> 
> > reference whereas the TDX.PAGE.ADD path [via kvm_gmem_populate()] does not
> > take a folio reference.
> > 
> > Why are these paths different?
> > 

[snip]