From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pl1-f198.google.com (mail-pl1-f198.google.com [209.85.214.198]) (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 6BBEC46EF9F for ; Fri, 14 Aug 2026 13:03:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.214.198 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786712639; cv=none; b=Rhd1cz7OfpCYJp/QbZASGiunpv49PpeswB7pdfKi0b9gPoROx4B0Su9JmyLxRkSqsk2fEHfERsMOzlNvelac5z/234DbIrVYkDn+fmIC/dYCuvsVbABKa1gDv21yTcPO9Dk894Grk9pRu/J9ruIs2QOWI9UwjIpYRYWmIgryMmE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786712639; c=relaxed/simple; bh=2tza/Lhp8sgCN48mxycDZ5jMIUVa2dXecs8Tu2/DIBQ=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=kkGpkELmqlj+HiOutSWMq1uQUJoQyXlYr3U2X8NLjosLGbUJVV4jHG0LzJ4Z5vQWkCTDqpFcK6Ui+tzr1LToTy0AaV6FjiIt79cc6YrH7zBE2nXnWjCaiAu5XJTNzOTecYwmiKjchSpC6ptaiNAj1fZZw6LJ1kiZ1spJE1Ufras= 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=ejji8xhI; arc=none smtp.client-ip=209.85.214.198 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="ejji8xhI" Received: by mail-pl1-f198.google.com with SMTP id d9443c01a7336-2cfca8558d2so14915925ad.2 for ; Fri, 14 Aug 2026 06:03:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1786712637; x=1787317437; 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=gZcYmV5Cug1VJQ3U1/VOA01nGHc5KRf249qTuRVD+sM=; b=ejji8xhIYLPKv0f+j1ZIJzPv8BttSNQxyAiEFWmbGuKTnao0Z6+I4+/rBpq33hR53R TPqnz0nSrJ98Zud9NTZ9+IM1RcEXRX9yyfIqhyuXl+g1n0x6yasLD3doawMWu15ciCpo +cqFjseIFEDYx1bLk58rWcJNwaLZK+XF1FMgHjsLyuccqjBaST4TsnE7iu8Qq1OXvcFl R19k62GRBn0wjYaFzfO+Qb++7z2IWkqldckntFrRVMJncMddk9xQoUWilUog7rf50/7f JlYl6dheQW0LqbZ9z1TOoLvCHrgxPk7u432+qKRySi/oo++BAfP066hSf8ueV2eqKfac +N6A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1786712637; x=1787317437; 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=gZcYmV5Cug1VJQ3U1/VOA01nGHc5KRf249qTuRVD+sM=; b=m4Y1SvuCCfFSSeV2QqU05VmG3LkFOK/XsufpaD33ceXk06lcrC8SqidX77KepEo2e+ Idc7CeXRmm+4Zam4slsLi3DDPjMreleeXuRFNbVcRkaCVJ1kqqPQCjtNn+vKzvM/ICqI aSl+Z7b/nK9UTXA9bXQRmTzTuvyuahZXD0abeh9Q+s6LSZ0pECPsCMxzmFYWWJCeAdqh rlqt9VgeLxR0ixUvrp4Zp9JP2lZUlyq/dqf9rzVOH4jLBO9SJE5JBBOT5T8LoGTIW6DT zoRU7X6uxb6kaz+nT2qAd7OoqfXC1bH4/dJmOopTmdjBLXXJu/aZaHSgKuFuoqxuhK6z nqLg== X-Forwarded-Encrypted: i=1; AHgh+Ro3sa3XUEZuLnifVhMzDziOW+wHbDeBfZPLMVXFOZooxBfL4YTdaEa1P6Fqqf+eiJgkHKlO/sw=@lists.linux.dev X-Gm-Message-State: AOJu0YzP/Kjh8voqbunDuR/dpbHA3mktCkcrumcEHoRtqtXtN9KdRxcE JEmNCD+i/zZWJZwTVIa2GorYiqVJlcYiafLdw7uHc5mbCqRk5DCBze3Szt9xd/4Jk7WvDVUIoVk ObBGFrA== X-Received: from pldv11.prod.google.com ([2002:a17:902:ca8b:b0:2c7:e06e:86f6]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:903:3204:b0:2c0:e2ea:6b0c with SMTP id d9443c01a7336-2d3b0d88bd1mr71228795ad.21.1786712636303; Fri, 14 Aug 2026 06:03:56 -0700 (PDT) Date: Fri, 14 Aug 2026 06:03:54 -0700 In-Reply-To: <487aa57c-72ad-453b-971d-ca24a8380429@arm.com> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <487aa57c-72ad-453b-971d-ca24a8380429@arm.com> Message-ID: Subject: Re: [RFC PATCH 0/3] KVM: Dirty page logging for guest_memfd-only memslots From: Sean Christopherson To: David Hildenbrand Cc: Alexandru Elisei , Mark Rutland , pbonzini@redhat.com, kvm@vger.kernel.org, maz@kernel.org, oupton@kernel.org, joey.gouly@arm.com, seiden@linux.ibm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, fuad.tabba@linux.dev Content-Type: text/plain; charset="us-ascii" On Fri, Aug 14, 2026, David Hildenbrand wrote: > We have a hardware feature that requires pages to always be mapped into S2. Some > things I had in mind: > > 1) Page migration would not be a problem as long as hardware could be paused > while migrating (e.g., kick all vCPUs). I doubt someone would implement that > right now, but you could consider it an implementation detail that page > migration cannot be supported right now. > > 2) Newer hardware could mitigate this problem, allowing the feature to support > pages temporarily being unmapped from S2. > > 3) Disallowing page migration is really just one implication of "pages must > always be mapped into S2". > > So what we really want is "if feature X is enabled and hardware requires it, > always keep pages mapped into S2, which currently implies that page migration > cannot be supported." > > Which isn't all that different to "if a confidential VM is run on current TDX > hardware, always keep pages mapped into S2, which currently implies that page > migration cannot be supported." > > So I was wondering whether the flow could be: > > User space enabled CPU feature for VM -> KVM knows that current hardware > requires for that CPU feature to have S2 always mapped -> KVM tells guest_memfd > that S2 must be always mapped / disables page migration. I'm a-ok with adding a flag to guest_memfd to communicate whether or not page migration is allowed, because guest_memfd needs to actively support page migration. I'm not ok adding a flag telling guest_memfd that memory must always be mapped in S2, because guest_memfd doesn't care. E.g. KVM doesn't yet support page migration for SNP, but SNP tracks page ownership in an out-of-band table and so KVM can map/unmap all guest memory from S2 at will. > That would be in contrast to user space having to guess that page migration on > the current hardware with the current guest_memfd implementation does not > support page migration, to then disable exactly that. > > Does that explanation makes sense? I don't know the exact mechanism to do that, > but that's just my high-level thinking. Yes, I'm supportive of KVM expressing to guest_memfd that page migration isn't supported by the VM. I'm only objecting to expressing that memory must stay mapped in S2, because guest_memfd doesn't care *why* page migration is or isn't supported/allowed by a particular VM.