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

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Message-ID: <aa1835299cfd79788572a212761d8838c388ed37.camel@intel.com>
Date: Thu, 28 Aug 2025 02:05:15 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "Annapurve, Vishal" <vannapurve@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"michael.roth@....com" <michael.roth@....com>, "Weiny, Ira"
	<ira.weiny@...el.com>
Subject: Re: [RFC PATCH 05/12] KVM: TDX: Drop superfluous page pinning in
 S-EPT management

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/

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.