linux-kernel - Re: [PATCH v4 04/28] KVM: x86/mmu: Add dedicated API to map guest

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Message-ID: <7e43061a-b901-4e8f-8ee9-57d67aaf4f38@linux.intel.com>
Date: Fri, 31 Oct 2025 15:58:22 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
 Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>,
 Huacai Chen <chenhuacai@...nel.org>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>, Anup Patel <anup@...infault.org>,
 Paul Walmsley <pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>,
 Albert Ou <aou@...s.berkeley.edu>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Janosch Frank <frankja@...ux.ibm.com>,
 Claudio Imbrenda <imbrenda@...ux.ibm.com>,
 Paolo Bonzini <pbonzini@...hat.com>, "Kirill A. Shutemov" <kas@...nel.org>,
 linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
 kvm@...r.kernel.org, loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org,
 linux-riscv@...ts.infradead.org, x86@...nel.org, linux-coco@...ts.linux.dev,
 linux-kernel@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>,
 Kai Huang <kai.huang@...el.com>, Michael Roth <michael.roth@....com>,
 Yan Zhao <yan.y.zhao@...el.com>, Vishal Annapurve <vannapurve@...gle.com>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>,
 Ackerley Tng <ackerleytng@...gle.com>
Subject: Re: [PATCH v4 04/28] KVM: x86/mmu: Add dedicated API to map
 guest_memfd pfn into TDP MMU

On 10/31/2025 4:09 AM, Sean Christopherson wrote:
> Add and use a new API for mapping a private pfn from guest_memfd into the
> TDP MMU from TDX's post-populate hook instead of partially open-coding the
> functionality into the TDX code.  Sharing code with the pre-fault path
> sounded good on paper, but it's fatally flawed as simulating a fault loses
> the pfn, and calling back into gmem to re-retrieve the pfn creates locking
> problems, e.g. kvm_gmem_populate() already holds the gmem invalidation
> lock.
>
> Providing a dedicated API will also removing several MMU exports that
> ideally would not be exposed outside of the MMU, let alone to vendor code.
> On that topic, opportunistically drop the kvm_mmu_load() export.  Leave
> kvm_tdp_mmu_gpa_is_mapped() alone for now; the entire commit that added
> kvm_tdp_mmu_gpa_is_mapped() will be removed in the near future.
>
> Gate the API on CONFIG_KVM_GUEST_MEMFD=y as private memory _must_ be backed
> by guest_memfd.  Add a lockdep-only assert to that the incoming pfn is
> indeed backed by guest_memfd, and that the gmem instance's invalidate lock
> is held (which, combined with slots_lock being held, obviates the need to
> check for a stale "fault").
>
> Cc: Michael Roth <michael.roth@....com>
> Cc: Yan Zhao <yan.y.zhao@...el.com>
> Cc: Ira Weiny <ira.weiny@...el.com>
> Cc: Vishal Annapurve <vannapurve@...gle.com>
> Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Reviewed-by: Kai Huang <kai.huang@...el.com>
> Link: https://lore.kernel.org/all/20250709232103.zwmufocd3l7sqk7y@amd.com
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Binbin Wu <binbin.wu@...ux.intel.com>