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

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Message-ID: <aLC5BReucEqmBNTr@google.com>
Date: Thu, 28 Aug 2025 13:16:05 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: Yan Y Zhao <yan.y.zhao@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, Vishal Annapurve <vannapurve@...gle.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"michael.roth@....com" <michael.roth@....com>, Ira Weiny <ira.weiny@...el.com>
Subject: Re: [RFC PATCH 05/12] KVM: TDX: Drop superfluous page pinning in
 S-EPT management

On Thu, Aug 28, 2025, Rick P Edgecombe wrote:
> On Wed, 2025-08-27 at 16:33 +0800, Yan Zhao wrote:
> > 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/

Oh, nice!  I'll grab that and massage the changelog to break the hard dependencies
on the rest of the hugepage support.

> Yea, so the outcome of the huge page related discussion was that we should look
> at some sort of emergency page reclaim feature for the TDX module to use in the
> case of bugs. But in the meantime to move forward without it, using a solution
> like in this patch.