From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53145) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eh4W3-0006GU-RZ for qemu-devel@nongnu.org; Wed, 31 Jan 2018 21:22:20 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eh4W0-00035z-LJ for qemu-devel@nongnu.org; Wed, 31 Jan 2018 21:22:19 -0500 Received: from mx1.redhat.com ([209.132.183.28]:49384) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1eh4W0-00035W-Ea for qemu-devel@nongnu.org; Wed, 31 Jan 2018 21:22:16 -0500 Date: Thu, 1 Feb 2018 04:22:07 +0200 From: "Michael S. Tsirkin" Message-ID: <20180201040608-mutt-send-email-mst@kernel.org> References: <20180117095421.124787-1-marcel@redhat.com> <20180117095421.124787-2-marcel@redhat.com> <20180131204059.GG21702@localhost.localdomain> <20180131230607-mutt-send-email-mst@kernel.org> <20180131233422.GP26425@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180131233422.GP26425@localhost.localdomain> 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: Eduardo Habkost 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 Wed, Jan 31, 2018 at 09:34:22PM -0200, Eduardo Habkost wrote: > On Wed, Jan 31, 2018 at 11:10:07PM +0200, Michael S. Tsirkin wrote: > > On Wed, Jan 31, 2018 at 06:40:59PM -0200, Eduardo Habkost wrote: > > > On Wed, Jan 17, 2018 at 11:54:18AM +0200, Marcel Apfelbaum wrote: > > > > Currently only file backed memory backend can > > > > be created with a "share" flag in order to allow > > > > sharing guest RAM with other processes in the host. > > > > > > > > Add the "share" flag also to RAM Memory Backend > > > > in order to allow remapping parts of the guest RAM > > > > to different host virtual addresses. This is needed > > > > by the RDMA devices in order to remap non-contiguous > > > > QEMU virtual addresses to a contiguous virtual address range. > > > > > > > > > > Why do we need to make this configurable? Would anything break > > > if MAP_SHARED was always used if possible? > > > > See Documentation/vm/numa_memory_policy.txt for a list > > of complications. > > Ew. > > > > > Maybe we should more of an effort to detect and report these > > issues. > > Probably. Having other features breaking silently when using > pvrdma doesn't sound good. We must at least document those > problems in the documentation for memory-backend-ram. > > BTW, what's the root cause for requiring HVAs in the buffer? It's a side effect of the kernel/userspace API which always wants a single HVA/len pair to map memory for the application. > Can > this be fixed? I think yes. It'd need to be a kernel patch for the RDMA subsystem mapping an s/g list with actual memory. The HVA/len pair would then just be used to refer to the region, without creating the two mappings. Something like splitting the register mr into mr = create mr (va/len) - allocate a handle and record the va/len addmemory(mr, offset, hva, len) - pin memory register mr - pass it to HW As a nice side effect we won't burn so much virtual address space. This will fix rdma with hugetlbfs as well which is currently broken. > -- > Eduardo