From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41495) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ehIo6-0002O9-8U for qemu-devel@nongnu.org; Thu, 01 Feb 2018 12:37:55 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ehIo2-0006mv-Al for qemu-devel@nongnu.org; Thu, 01 Feb 2018 12:37:54 -0500 Received: from mx1.redhat.com ([209.132.183.28]:59142) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ehIo2-0006gs-1r for qemu-devel@nongnu.org; Thu, 01 Feb 2018 12:37:50 -0500 Date: Thu, 1 Feb 2018 15:36:55 -0200 From: Eduardo Habkost Message-ID: <20180201173655.GC26425@localhost.localdomain> References: <8dbc7c99-84f6-0023-526b-359fdf2b5162@redhat.com> <20180201121009.GR26425@localhost.localdomain> <20180201150254-mutt-send-email-mst@kernel.org> <20180201163132.GV26425@localhost.localdomain> <20180201184655-mutt-send-email-mst@kernel.org> <20180201165739.GY26425@localhost.localdomain> <20180201185830-mutt-send-email-mst@kernel.org> <20180201170136.GA26425@localhost.localdomain> <20180201190902-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180201190902-mutt-send-email-mst@kernel.org> Subject: Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: Marcel Apfelbaum , qemu-devel@nongnu.org, cohuck@redhat.com, f4bug@amsat.org, yuval.shaia@oracle.com, borntraeger@de.ibm.com, pbonzini@redhat.com, imammedo@redhat.com On Thu, Feb 01, 2018 at 07:12:45PM +0200, Michael S. Tsirkin wrote: > On Thu, Feb 01, 2018 at 03:01:36PM -0200, Eduardo Habkost wrote: > > On Thu, Feb 01, 2018 at 06:59:07PM +0200, Michael S. Tsirkin wrote: > > > On Thu, Feb 01, 2018 at 02:57:39PM -0200, Eduardo Habkost wrote: > > > > On Thu, Feb 01, 2018 at 06:48:54PM +0200, Michael S. Tsirkin wrote: > > > > > On Thu, Feb 01, 2018 at 02:31:32PM -0200, Eduardo Habkost wrote: > > > > > > On Thu, Feb 01, 2018 at 04:24:30PM +0200, Michael S. Tsirkin wrote: > > > > > > [...] > > > > > > > The full fix would be to allow QEMU to map a list of > > > > > > > pages to a guest supplied IOVA. > > > > > > > > > > > > Thanks, that's what I expected. > > > > > > > > > > > > While this is not possible, the only requests I have for this > > > > > > patch is that we clearly document: > > > > > > * What's the only purpose of share=on on a host-memory-backend > > > > > > object (due to pvrdma limitations). > > > > > > * The potential undesirable side-effects of setting share=on. > > > > > > * On the commit message and other comments, clearly distinguish > > > > > > HVAs in the QEMU address-space from IOVAs, to avoid confusion. > > > > > > > > > > Looking forward, when we do support it, how will management find out > > > > > it no longer needs to pass the share parameter? > > > > > > > > > > Further, if the side effects of the share parameter go away, > > > > > how will it know these no longer hold? > > > > > > > > A query-host-capabilities or similar QMP command seems necessary > > > > for that. > > > > > > Is anyone working on that? > > > > Not yet. > > > > -- > > Eduardo > > Do these patches need to wait until we do have that command? I don't think so. The command will be needed only when support for pvrdma without share=on gets implemented. Right now, all we need is clear documentation. > > I'm thinking it's better to have "share=on required with rdma" > and "hugetlbfs not supported with rdma" > than the reverse, this way new hosts do not need to carry > thus stuff around forever. What do you mean by "the reverse"? IIUC, the requirements/limitations are: * share=on required for pvrdma. Already documented and enforced by pvrdma code in this series. * hugetlbfs not supported with rdma. Is this detected/reported by QEMU? Is it documented? * side-effects of share=on. This is not detected nor documented, and probably already applies to other memory backends. * Nice to have: document when share=on is useful (answer: because of pvrdma), when adding share=on support to host-memory-backend. > > Also, how does management know which devices are affected? Right now? By reading documentation. -- Eduardo