From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2D8B01993B2 for ; Thu, 16 Jan 2025 20:19:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737058800; cv=none; b=KoXqiQskHp32BeP4hZJ18C2pFWLt9HY86RXCIMSDMLZC+dtyQGYcGMOarbXy2pxZz1uMxYjOE0RXumMYEbg3rNt8agxGabBL9WTLvDHKpJTSBIbJlW5SD0Xjfjs9xg/sVzHbdpdyKYHgl7JBUT6bRbQUFjYrMLqU8Z5om7+ggeQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737058800; c=relaxed/simple; bh=nNoBF4UfWoe23a1lrQ7UdZ6UWanGe/SAkh9O53HaEmA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=cv4xqJ6Z2lmIunYAppG34mc+NoIZhaIm77XfpgGCZ6MdxpcyFsulFof3RcaADskKSl2ulFGiGPkj63bUWz/4QCzR8kJ6cjVS0X51MlvfsAc+6YlGUPBFHjKzX3H2RXA8+k6h6wfd38xdWSblZSyMGdcJpX/LJY6vlzjZako+WaU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=Gj069CJa; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Gj069CJa" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1737058798; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=hxyx17H16LTaCMPogGfVhxD8KdAjR1MUqsEp+1MdmBk=; b=Gj069CJa4nUwP9pKLTx2K5xZxYQESBVIoikqa9aC/KEUu5iNOYUOCzLH/zic+lmPFyOIga G9zjhhgA69K46lgaCiVTQoh4F4qJ0+/ow4VSS3XpEB5sBDmXlFB8f3YoEKIAyKnTUoTcOJ 7AkXARApq90qXGCo36r3AybiBSQqNPQ= Received: from mail-pl1-f197.google.com (mail-pl1-f197.google.com [209.85.214.197]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-659-fHPyotw-NMyQzjzOCDaB3g-1; Thu, 16 Jan 2025 15:19:56 -0500 X-MC-Unique: fHPyotw-NMyQzjzOCDaB3g-1 X-Mimecast-MFC-AGG-ID: fHPyotw-NMyQzjzOCDaB3g Received: by mail-pl1-f197.google.com with SMTP id d9443c01a7336-2164861e1feso23830605ad.1 for ; Thu, 16 Jan 2025 12:19:56 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1737058795; x=1737663595; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=hxyx17H16LTaCMPogGfVhxD8KdAjR1MUqsEp+1MdmBk=; b=lRKfGHEuqjSBhjtnihkIm398QaX3d+p6HVn91Gb/4Xe+RkEz0OIumZp4wXCBWuXL2P EVzD99oTzi43FNIR4tpCZ7tR1Q6+7Yj0uHnZAYTFSessouDvSyXwsNrsqPilxFtmTXtx qMiN9S5bgRO94BpHjR7tyf9fsgK53oJJe4un5ER8r7AG1DeC9mugAQdqU/Dy1GL2WUgs vbf/mR6oDSZg89CX8yY/X6EkHa0RsaIEttDjzP/Ez26+bfwZJkcBwAyoBXsrL3u7+WZj cCs6gX52OOrPTbXkR3CBD8taVLtKrf/suFCXeDJbJOw8in4x5l5C8S45uRFMjsuP8UpY yMUg== X-Forwarded-Encrypted: i=1; AJvYcCVY16EW7hMZoaSevPp4x81SsU6JNiXdEk+Ba5bqR52asDTzC01iK2rrux6vhCJ35Uv4Plg=@vger.kernel.org X-Gm-Message-State: AOJu0YxHUnSEwrHD70E/sMBgxFUWlKQOA9MHSF78HtWihUzK0X+lSWuT UdDQLDDtHvsbdfc/bXB/P6//W1PHSCR75bWeUieBJ3Ex9tNgNKqf/zh6Tk6aAygruNa+uUpEV0j 3CHN+BdVyOm9a9m7gu5Y/p46hijwzGGjeSv/gM8j+JCrVPNlWtA== X-Gm-Gg: ASbGncu8cxkyln+kY2eUAEoe91efGwigFC2y5mwb/bMhIX9I0DZIat3P2yx3+qoZOeZ k2Yb8IRiZwsFFxo3W/iUT3qJ4y81Tnf3FdgEWcFs6ULPi0dYnZGnZNr1lL9SxRXXW33m3F/ULm4 ag/wjbpUguI0vsTIliCWOAXHmXNkTsSLLGtkA5ffln7t7e1X3ldxibXV087pEphpnUdXXcLpgYH fkbKT86GTljtmwZQ90ySHvuv1KtS/zkF5APWzi6Rou5Pyz0ufOlgRzStnnU6tO7K/yZ5eVKvzhr SV6SiX1NDhDlQQPpKQ== X-Received: by 2002:a17:903:1105:b0:216:6f1a:1c81 with SMTP id d9443c01a7336-21c352c77eamr1589175ad.2.1737058795385; Thu, 16 Jan 2025 12:19:55 -0800 (PST) X-Google-Smtp-Source: AGHT+IFCdUZtxdruuZ9dhKBDB1S+oAh8pcb12HPEW8IyTWaMSLHmWxJL1Esm0PAWT5jrhgt1RJfKYw== X-Received: by 2002:a17:903:1105:b0:216:6f1a:1c81 with SMTP id d9443c01a7336-21c352c77eamr1588855ad.2.1737058795018; Thu, 16 Jan 2025 12:19:55 -0800 (PST) Received: from x1n (pool-99-254-114-190.cpe.net.cable.rogers.com. [99.254.114.190]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-21c2d3ac3e2sm3791465ad.139.2025.01.16.12.19.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 16 Jan 2025 12:19:54 -0800 (PST) Date: Thu, 16 Jan 2025 15:19:49 -0500 From: Peter Xu To: James Houghton Cc: Paolo Bonzini , Sean Christopherson , Jonathan Corbet , Marc Zyngier , Oliver Upton , Yan Zhao , Nikita Kalyazin , Anish Moorthy , Peter Gonda , David Matlack , Wei W , kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev Subject: Re: [PATCH v1 00/13] KVM: Introduce KVM Userfault Message-ID: References: <20241204191349.1730936-1-jthoughton@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: James, Sorry for a late reply. I still do have one or two pure questions, but nothing directly relevant to your series. On Thu, Jan 02, 2025 at 12:53:11PM -0500, James Houghton wrote: > So I'm not pushing for KVM Userfault to replace userfaultfd; it's not > worth the extra/duplicated complexity. And at LPC, Paolo and Sean > indicated that this direction was indeed wrong. I have another way to > make this work in mind. :) Do you still want to share it, more or less? :) > > For the gmem case, userfaultfd cannot be used, so KVM Userfault isn't > replacing it. And as of right now anyway, KVM Userfault *does* provide > a complete post-copy system for gmem. > > When gmem pages can be mapped into userspace, for post-copy to remain > functional, userspace-mapped gmem will need userfaultfd integration. > Keep in mind that even after this integration happens, userfaultfd > alone will *not* be a complete post-copy solution, as vCPU faults > won't be resolved via the userspace page tables. Do you know in context of CoCo, whether a private page can be accessed at all outside of KVM? I think I'm pretty sure now a private page can never be mapped to userspace. However, can another module like vhost-kernel access it during postcopy? My impression of that is still a yes, but then how about vhost-user? Here, the "vhost-kernel" part represents a question on whether private pages can be accessed at all outside KVM. While "vhost-user" part represents a question on whether, if the previous vhost-kernel question answers as "yes it can", such access attempt can happen in another process/task (hence, not only does it lack KVM context, but also not sharing the same task context). Thanks, -- Peter Xu