From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.4 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,T_HK_NAME_DR,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E31BDC65BAE for ; Thu, 13 Dec 2018 20:09:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B35C820879 for ; Thu, 13 Dec 2018 20:09:45 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B35C820879 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727104AbeLMUJo (ORCPT ); Thu, 13 Dec 2018 15:09:44 -0500 Received: from mx1.redhat.com ([209.132.183.28]:51348 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725949AbeLMUJo (ORCPT ); Thu, 13 Dec 2018 15:09:44 -0500 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 3A000307EA90; Thu, 13 Dec 2018 20:09:44 +0000 (UTC) Received: from work-vm (ovpn-117-213.ams2.redhat.com [10.36.117.213]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EE4FE19744; Thu, 13 Dec 2018 20:09:38 +0000 (UTC) Date: Thu, 13 Dec 2018 20:09:36 +0000 From: "Dr. David Alan Gilbert" To: Dan Williams Cc: vgoyal@redhat.com, linux-fsdevel , Linux Kernel Mailing List , KVM list , Miklos Szeredi , stefanha@redhat.com, sweil@redhat.com, swhiteho@redhat.com Subject: Re: [PATCH 15/52] fuse: map virtio_fs DAX window BAR Message-ID: <20181213200936.GU2313@work-vm> References: <20181210171318.16998-1-vgoyal@redhat.com> <20181210171318.16998-16-vgoyal@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Thu, 13 Dec 2018 20:09:44 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Dan Williams (dan.j.williams@intel.com) wrote: > On Mon, Dec 10, 2018 at 9:22 AM Vivek Goyal wrote: > > > > From: Stefan Hajnoczi > > > > Experimental QEMU code introduces an MMIO BAR for mapping portions of > > files in the virtio-fs device. Map this BAR so that FUSE DAX can access > > file contents from the host page cache. > > FUSE DAX sounds terrifying, can you explain a bit more about what this is? We've got a guest running in QEMU, it sees an emulated PCI device; that runs a FUSE protocol over virtio on that PCI device, but also has a trick where via commands sent over the virtio queue associated with that device, (fragments of) host files get mmap'd into the qemu virtual memory that corresponds to the kvm slot exposed to the guest for that bar. The guest sees those chunks in that BAR, and thus you can read/write to the host file by directly writing into that BAR. > > The DAX window is accessed by the fs/dax.c infrastructure and must have > > struct pages (at least on x86). Use devm_memremap_pages() to map the > > DAX window PCI BAR and allocate struct page. > > PCI BAR space is not cache coherent, Note that no real PCI infrastructure is involved - this is all emulated devices, backed by mmap'd files on the host qemu process. Dave > what prevents these pages from > being used in paths that would do: > > object = page_address(pfn_to_page(virtio_fs_pfn)); > > ...? -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK