From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pg1-f200.google.com (mail-pg1-f200.google.com [209.85.215.200]) (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 EB043377ED7 for ; Tue, 18 Aug 2026 19:55:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.215.200 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787082948; cv=none; b=hsoIbUTwMl0hFw9WCejP1F+7PyQKk2UDKj1elDMZgR/rLVFTL6aZ4RWQAg/qdRbOZGX1Ra5VtCi/3fPzh7tSNtakEokmXm+bK2dQAF5zZvl5eLOcqyZWSAnvzGUIrdrROhXtgr+jWH/GRr9EAGhT2uApmce71mpnrmiNjCC4Z1Q= 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=bKSKv/9v; arc=none smtp.client-ip=209.85.215.200 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="bKSKv/9v" Received: by mail-pg1-f200.google.com with SMTP id 41be03b00d2f7-cbbb9c9bfc3so100684a12.1 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=vger.kernel.org; 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=bKSKv/9vp2FNvYmNL/CPWvS2mVwzhaAZh0qd/nczcKSoS+hCxoJRh3ZZh2M6n3wveN DnF4cVi0OUL4hwkdB3bJGDa64trOw6nyCctDqXOjNHyYlzVACGOaqj45wgTCgnom6o9M lvqX40ioACfnko/JzNBE1SIYk+qA6Z9eZ5M50wGgS+qOdNcVt7PMBn9DHzetwAMIq7gi lRtrxQ8RFlVT8LDpgZQ/iQZdNuK0fPI9Tql70aS33aH6T3HXrq3z4dvB/wdevzc/q/E5 7OKLDpJfmSu65bcYk02/qw5rcirqhw/H5AAcZu3QgBzSpb+5z+PQpVggfmwbc3ZYIjNf yb1Q== 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=BC2mkzVKoJ+PRypBqh6L2oOtE2MItIqHGfqETni9fxBbSqwgpV/e4uXHIuD6Sh+Llo NcM/MhL64ljDfTa9+uw7U31pvuq1ulgMQXm06hi/M1MQiabXXTLeWQ8ehTBnpajTAsVG cGf6QF2V4yoInIzB3FlDKTzNA9Vi8gy36ZoecgY4Qwo702CeY0ZPrMzz8cMpqZKCAbgS l3+Ppth6pba7JBoGj5kLqkhgu7jSVo89oLMON1vsOrzwbMFEgYXTXMBZ9x+TkyGfaD3M gWOpsAjQr3ekfwtiEl14fK6c4WTdMgulcb/4dGSlAGwgwN9sJoF2pO3u/2VWBjLE29NF UgXw== X-Forwarded-Encrypted: i=1; AHgh+RppaUZqiw7xxX1pNRx7/xAEzwT8rn8U0JxaeDRev26o0uVsLtG7jwr2XFelKw69m9zWPmg=@vger.kernel.org X-Gm-Message-State: AOJu0Yyo0YwfeX84ndUGhREz8Fq90QtX2nh16x2mMAtSl4291LYZJHmu TTbG1uHjHnnPJtMStgy48jw2FG2jLOyvMtXhXFS8PIB6hkjSccslDALSFzppReNAMRs1TNr7Bek mrc/ssA== 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: kvm@vger.kernel.org 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.