From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pg1-f197.google.com (mail-pg1-f197.google.com [209.85.215.197]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C42281D95A3 for ; Tue, 18 Aug 2026 19:55:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.215.197 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787082948; cv=none; b=hxb2EElTAFrsPeqKHpEo7yWGuJzgzNrItbgCgv/mZb3Y+lwSqPe1kF1x2A0DjYCQvcktvXHJ6mM+1vvjN3RHS8/ofV+FZR5x9Ic/TBaOqMSJlUTZjcV+qG+ajXAcqHcFlK/qubY6fxo4esSHU7q47/AdxQ+e82p8yGEJWznGjSs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787082948; c=relaxed/simple; bh=iCso+0ky8Dzh3YExOIjLuI1F1ucg1k8ItKeGzglIZmA=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=UtjBYaviK2rOtsSXB3UEUn2kP2mOmetAAxAwfqGzcdNRQQ9IMQv/Zq0dbHlMJZTa6jetj9DA4nT8T5LIwzRLXv5wZEeFCXkvsrOHhaYiMK+L0W5MMC/Oki4q3aMlYcBtR1y1YPTSGoz5eW5Upw6Lv6J4Y+IMzbziPDnhDfzmm8I= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=R1TZVxBo; arc=none smtp.client-ip=209.85.215.197 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="R1TZVxBo" Received: by mail-pg1-f197.google.com with SMTP id 41be03b00d2f7-cab3e9cd922so104679a12.0 for ; Tue, 18 Aug 2026 12:55:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1787082946; x=1787687746; darn=lists.linux.dev; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:from:to:cc:subject:date:message-id:reply-to :content-type; bh=kuUJP/9WbIJ+dwbEGRfTcEg4pKKbPnOHED5niMcUDBA=; b=R1TZVxBo/WoYtSKGNYBzkbN3pvoTQXNplKj4boaGX8SUOZE0NfP46pCk6p+XEvDbNn r5K4nohiqRbeYoMY1E+G9zGkg7biRw9+nb4/ddYm5yqvOLAYoiX+KAGwnHApZeydgj78 VstuFCiD7gWM3YLk5kAyhlu67rbWbRfx/2jB+WAMKJG1Y7P0vsfmUbDXqNy4DwppBgiI fHl/l6nTTaBLTFmOW7rsFHCBU4JUVs2XpkJqXuwiOyPkEiSECJcsh/jDHENQtGZz8zkA ybrGfD4udSkEK99vCAmT9AaDhJ/AcKVhj/prGm08lOUSqe/JuRTV9IM8v+Ino/Ygf/Ee Alug== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1787082946; x=1787687746; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to:content-type; bh=kuUJP/9WbIJ+dwbEGRfTcEg4pKKbPnOHED5niMcUDBA=; b=ma2C08iiYK70Le2WI0EH/0w5NoQZP2JOaQyVhPK2j+JR/sGe6AUSGUVwD49uffPwXG uDQ2iY/SJoNWgan6dqPro3jEHCHsD6jqsWUsQyp2J0NUUO4CRDyNWPfkw2J6gpcUPxIs iDDroRBk0dwUTAgYL6R1/exlMztTWsMXs9iOu7Jd/Q5L9lYD0kYjq1EItI4NjG6C/scJ h+KD5oGAHwyaUtPub7yAwmFP9QXvxDnsqyckeUFUNMbcH/U7DykyB95kis4StqDcsvLM oP4HsmOtL7Aw2+yoFQi65Y8QHfAv93PdAzC+yRIlPiSPQ99+d+6cvmvGrQ7MvRR5LwaR xclQ== X-Forwarded-Encrypted: i=1; AHgh+RpZwjodSKKrBSrhDV+NDGwv/jUqVrL6CTgHsRESsoBhY/qieLCNJsbqTS9LUMWS5PoMFU0zCe8=@lists.linux.dev X-Gm-Message-State: AOJu0Yx5TVTugSrlmKCaHTET7JH1Bt0sJ9cIhoWJgZS4RzmSDZpGJHYF PIEj1lqUAPQUJnrdQJ0Er+QkpwcE+nzMXner2iktRenE87q0CLziQxp/73C8gsByyvPUGIlkMrv gaBQdrg== X-Received: from pglg36.prod.google.com ([2002:a63:1124:0:b0:cc1:522f:464c]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a20:cd96:b0:3bf:6d96:ac40 with SMTP id adf61e73a8af0-3ccfef7af73mr418448637.12.1787082945745; Tue, 18 Aug 2026 12:55:45 -0700 (PDT) Date: Tue, 18 Aug 2026 12:55:45 -0700 In-Reply-To: <807b4b9f-b1ca-4b5c-bc9e-22bf0637ba33@kernel.org> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260818-gmem-no-return-page-v2-0-5298f42d49bb@google.com> <807b4b9f-b1ca-4b5c-bc9e-22bf0637ba33@kernel.org> Message-ID: Subject: Re: [PATCH v2 0/4] Stop returning struct page from guest_memfd PFN lookup From: Sean Christopherson To: "David Hildenbrand (Arm)" Cc: Ackerley Tng , Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Ashish Kalra , Michael Roth , Brijesh Singh , Marc Zyngier , Oliver Upton , Joey Gouly , Steffen Eiden , Suzuki K Poulose , Zenghui Yu , Catalin Marinas , Will Deacon , Fuad Tabba , Yan Zhao , Rick P Edgecombe , Vishal Annapurve , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev Content-Type: text/plain; charset="us-ascii" On Tue, Aug 18, 2026, David Hildenbrand (Arm) wrote: > On 8/18/26 11:15, Ackerley Tng wrote: > > KVM currently expects kvm_gmem_get_pfn() to return a refcounted struct > > page. Callers (such as x86 TDP MMU, arm64 Stage-2 fault handler, and SEV-SNP > > VMSA / RMP handlers) hold this refcount across page fault handling. > > > > Holding a page refcount across fault handling is problematic for guest_memfd. > > In-place memory conversions between confidential computing shared and private > > states inspect folio refcounts to ensure exclusive ownership by guest_memfd. A > > concurrent guest page fault taking a reference on the folio causes conversions > > to fail due to an elevated refcount. > > Right. Won't we still, at least temporarily, grab a reference while looking up > the folio in the page cache, or will we be preventing that concurrent race with > locking? The latter. What I want to aim for is that if the relevant guest_memfd range has never been mmap()'d and there are no memory failures, then conversion is guaranteed to not fail due to elevated refcounts. Or to put it a different way, I want KVM's ABI to be that pausing vCPU is *NOT* required to perform an in-place conversion.