From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f70.google.com (mail-wm0-f70.google.com [74.125.82.70]) by kanga.kvack.org (Postfix) with ESMTP id A6ABE6B03A0 for ; Mon, 19 Jun 2017 06:08:18 -0400 (EDT) Received: by mail-wm0-f70.google.com with SMTP id d184so11375726wmd.15 for ; Mon, 19 Jun 2017 03:08:18 -0700 (PDT) Received: from mail-wm0-x232.google.com (mail-wm0-x232.google.com. [2a00:1450:400c:c09::232]) by mx.google.com with ESMTPS id s12si6045172wmd.7.2017.06.19.03.08.17 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 19 Jun 2017 03:08:17 -0700 (PDT) Received: by mail-wm0-x232.google.com with SMTP id d73so68976422wma.0 for ; Mon, 19 Jun 2017 03:08:17 -0700 (PDT) Date: Mon, 19 Jun 2017 11:08:13 +0100 From: Stefan Hajnoczi Subject: Re: [RFC] virtio-mem: paravirtualized memory Message-ID: <20170619100813.GB17304@stefanha-x1.localdomain> References: <547865a9-d6c2-7140-47e2-5af01e7d761d@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="ADZbWkCsHQ7r3kzd" Content-Disposition: inline In-Reply-To: <547865a9-d6c2-7140-47e2-5af01e7d761d@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: David Hildenbrand Cc: KVM , "virtualization@lists.linux-foundation.org" , "qemu-devel@nongnu.org" , "linux-mm@kvack.org" , Andrea Arcangeli , "Michael S. Tsirkin" --ADZbWkCsHQ7r3kzd Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Fri, Jun 16, 2017 at 04:20:02PM +0200, David Hildenbrand wrote: > Important restrictions of this concept: > - Guests without a virtio-mem guest driver can't see that memory. > - We will always require some boot memory that cannot get unplugged. > Also, virtio-mem memory (as all other hotplugged memory) cannot become > DMA memory under Linux. So the boot memory also defines the amount of > DMA memory. I didn't know that hotplug memory cannot become DMA memory. Ouch. Zero-copy disk I/O with O_DIRECT and network I/O with virtio-net won't be possible. When running an application that uses O_DIRECT file I/O this probably means we now have 2 copies of pages in memory: 1. in the application and 2. in the kernel page cache. So this increases pressure on the page cache and reduces performance :(. Stefan --ADZbWkCsHQ7r3kzd Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEcBAEBAgAGBQJZR6KNAAoJEJykq7OBq3PIQqMH/0S2AZbkzrqnJOKdCz8yKg9X qBbZG8McVs38XBZtkkzU4J4JKQMsS7b+boDjJ8N5LmjuHrNFfnJrwScVVc8aQ++E muHRRed4s556aSBAcSvk/OT7CtxYdwrraFuvzp2O1Rt84m9RPrMv719xZxeWsbzo 69e7xRq3NIAgv2zLbPIWV/RiqXJIYWJatGP95n0PvKIeRwxl8jK68BUdEchWGjAQ mcjgcM8gch/facSMSd0OcRR4IgCLKuEV3RqKXJ2WKy//xWIjQc95m0sQEFWdwBtB Eri7GCsR0SDsOfGtoMmBvuQOePISkS1YiqNNLcSVKY2WJ5agzszR/rnFl6vvAF0= =zROB -----END PGP SIGNATURE----- --ADZbWkCsHQ7r3kzd-- -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org