From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47756) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cGyw9-0005Vp-8y for qemu-devel@nongnu.org; Tue, 13 Dec 2016 21:04:54 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cGyw5-00089g-A9 for qemu-devel@nongnu.org; Tue, 13 Dec 2016 21:04:53 -0500 Received: from mx1.redhat.com ([209.132.183.28]:54126) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cGyw5-00089L-51 for qemu-devel@nongnu.org; Tue, 13 Dec 2016 21:04:49 -0500 Date: Wed, 14 Dec 2016 10:04:43 +0800 From: Peter Xu Message-ID: <20161214020443.GF32222@pxdev.xzpeter.org> References: <1481613903-17467-1-git-send-email-peterx@redhat.com> <1481613903-17467-3-git-send-email-peterx@redhat.com> <20161213064028.78938d07@t450s.home> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20161213064028.78938d07@t450s.home> Subject: Re: [Qemu-devel] [PATCH for-2.9 v2 2/2] intel_iommu: provide "aw-bits" parameter List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Alex Williamson Cc: qemu-devel@nongnu.org, kevin.tian@intel.com, famz@redhat.com, mst@redhat.com, jasowang@redhat.com On Tue, Dec 13, 2016 at 06:40:28AM -0700, Alex Williamson wrote: > On Tue, 13 Dec 2016 15:25:03 +0800 > Peter Xu wrote: > > > Previously vt-d codes only supports 39 bits iova address width. This > > patch provide a new parameter for Intel IOMMU to extend its address > > width to 48 bits. > > > > After enabling larger address width (48), we should be able to map > > larger iova addresses in the guest. To check whether 48 bits aw is > > enabled, we can grep in the guest dmesg with line: "dmar: Host address > > width 48". > > > > To keep compatibility with old QEMU and most hosts, default aw bits are > > set to 39 bits. > > I really fail to see the benefit of this option. Clearly we always > need to be able to support identity maps through the IOMMU, so the > address width of the vIOMMU needs to support the maximum address of any > memory or mmio range available to the VM. However if that's the case, > then the address width of the vIOMMU should be automatically configured > for the VM configuration. What's the benefit of creating yet another > obscure device option rather than configuring this automatically. If > the argument is to be able to test 48bit vIOMMU support, then we could > simply prefix this option with 'x-' to indicate an experimental, > unsupported option. Creating this option with the expectation that the > user must specify this in order to potentially create a valid VT-d > configuration for the VM doesn't make sense to me. Thanks, I'll rename with "x-" prefix and repost. Thanks, -- peterx