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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=unavailable 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 46198C43444 for ; Mon, 14 Jan 2019 10:17:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 22E1920659 for ; Mon, 14 Jan 2019 10:17:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726586AbfANKQ6 (ORCPT ); Mon, 14 Jan 2019 05:16:58 -0500 Received: from mx1.redhat.com ([209.132.183.28]:33476 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726187AbfANKQ6 (ORCPT ); Mon, 14 Jan 2019 05:16:58 -0500 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 0B79C7F6C5; Mon, 14 Jan 2019 10:16:57 +0000 (UTC) Received: from colo-mx.corp.redhat.com (colo-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.21]) by smtp.corp.redhat.com (Postfix) with ESMTPS id BA1525C25D; Mon, 14 Jan 2019 10:16:56 +0000 (UTC) Received: from zmail21.collab.prod.int.phx2.redhat.com (zmail21.collab.prod.int.phx2.redhat.com [10.5.83.24]) by colo-mx.corp.redhat.com (Postfix) with ESMTP id 39C333F600; Mon, 14 Jan 2019 10:16:56 +0000 (UTC) Date: Mon, 14 Jan 2019 05:16:55 -0500 (EST) From: Pankaj Gupta To: Jan Kara Cc: Dan Williams , KVM list , David Hildenbrand , linux-nvdimm , Jason Wang , Dave Chinner , Qemu Developers , virtualization@lists.linux-foundation.org, adilger kernel , Ross Zwisler , dave jiang , darrick wong , vishal l verma , "Michael S. Tsirkin" , Matthew Wilcox , Christoph Hellwig , Linux ACPI , jmoyer , linux-ext4 , Rik van Riel , Stefan Hajnoczi , Igor Mammedov , lcapitulino@redhat.com, Kevin Wolf , Nitesh Narayan Lal , Theodore Ts'o , xiaoguangrong eric , "Rafael J. Wysocki" , Linux Kernel Mailing List , linux-xfs , linux-fsdevel , Paolo Bonzini Message-ID: <2011860338.64100587.1547461015822.JavaMail.zimbra@redhat.com> In-Reply-To: <20190114095520.GC13316@quack2.suse.cz> References: <20190109144736.17452-1-pagupta@redhat.com> <20190110012617.GA4205@dastard> <20190110101757.GC15790@quack2.suse.cz> <1354249849.63357171.1547343519970.JavaMail.zimbra@redhat.com> <540171952.63371441.1547345866585.JavaMail.zimbra@redhat.com> <20190114095520.GC13316@quack2.suse.cz> Subject: Re: [Qemu-devel] [PATCH v3 0/5] kvm "virtio pmem" device MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [10.67.116.53, 10.4.195.7] Thread-Topic: kvm "virtio pmem" device Thread-Index: k+ecF0+cWZghQYre2RFLCXGyicEhfg== X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Mon, 14 Jan 2019 10:16:57 +0000 (UTC) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Message-ID: <20190114101655.8PpkPsn2HAk6dW_PO5P1lrsf6qDpfLdfiDiBub0glZ4@z> > > > > > Right. Thinking about this I would be more concerned about the fact > > > > > that > > > > > guest can effectively pin amount of host's page cache upto size of > > > > > the > > > > > device/file passed to guest as PMEM, can't it Pankaj? Or is there > > > > > some > > > > > QEMU > > > > > magic that avoids this? > > > > > > > > Yes, guest will pin these host page cache pages using 'get_user_pages' > > > > by > > > > elevating the page reference count. But these pages can be reclaimed by > > > > host > > > > at any time when there is memory pressure. > > > > > > Wait, how can the guest pin the host pages? I would expect this to > > > happen only when using vfio and device assignment. Otherwise, no the > > > host can't reclaim a pinned page, that's the whole point of a pin to > > > prevent the mm from reclaiming ownership. > > > > yes. You are right I just used the pin word but it does not actually pin > > pages > > permanently. I had gone through the discussion on existing problems with > > get_user_pages and DMA e.g [1] to understand Jan's POV. It does mention GUP > > pin pages so I also used the word 'pin'. But guest does not permanently pin > > these pages and these pages can be reclaimed by host. > > OK, then I was just confused how virtio-pmem is going to work. Thanks for > explanation! So can I imagine this as guest mmaping the host file and > providing the mapped range as "NVDIMM pages" to the kernel inside the > guest? Or is it more complex? yes, that's correct. Host's Qemu process virtual address range is used as guest physical address and a direct mapping(EPT/NPT) is established. At guest side, this physical memory range is plugged into guest system memory map and DAX mapping is setup using nvdimm calls. Thanks, Pankaj