From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:49108) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bjJZ3-0000w3-8Q for qemu-devel@nongnu.org; Mon, 12 Sep 2016 01:13:54 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bjJYy-0000Be-2z for qemu-devel@nongnu.org; Mon, 12 Sep 2016 01:13:53 -0400 Received: from mx1.redhat.com ([209.132.183.28]:33760) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bjJYx-0000BR-RK for qemu-devel@nongnu.org; Mon, 12 Sep 2016 01:13:48 -0400 Date: Mon, 12 Sep 2016 13:13:41 +0800 From: Peter Xu Message-ID: <20160912051341.GB3776@pxdev.xzpeter.org> References: <20160906052733.GA21051@pxdev.xzpeter.org> <92b5128b-d6e2-5787-9cea-07fcf90d22a5@redhat.com> <20160906081737.GE21051@pxdev.xzpeter.org> <8a6c53f9-e3c4-f788-e794-30f28c184418@redhat.com> <20160906103142.GG21051@pxdev.xzpeter.org> <20160907054419.GN2780@voom.fritz.box> <20160907063419.GI21051@pxdev.xzpeter.org> <20160907064154.GR2780@voom.fritz.box> <20160908090732.GB28348@pxdev.xzpeter.org> <20160912012603.GD12621@voom.fritz.box> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20160912012603.GD12621@voom.fritz.box> Subject: Re: [Qemu-devel] [PATCH 2/3] memory: add iommu_notify_flag List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: David Gibson Cc: Paolo Bonzini , mst@redhat.com, jasowang@redhat.com, qemu-devel@nongnu.org, cornelia.huck@de.ibm.com, alex.williamson@redhat.com, wexu@redhat.com, vkaplans@redhat.com, dgibson@redhat.com On Mon, Sep 12, 2016 at 11:26:04AM +1000, David Gibson wrote: > On Thu, Sep 08, 2016 at 05:07:32PM +0800, Peter Xu wrote: > > On Wed, Sep 07, 2016 at 04:41:54PM +1000, David Gibson wrote: > > > On Wed, Sep 07, 2016 at 02:34:19PM +0800, Peter Xu wrote: > > > > On Wed, Sep 07, 2016 at 03:44:19PM +1000, David Gibson wrote: > > > > > > For "CHANGE", it sounds like a unmap() + a map(). However I'd say > > > > > > "ADDITION" is nowhere better... > > > > > > > > > > Right.. this brings up a good point. > > > > > > > > > > Changing a mapping (i.e. overwriting an existing mapping with a > > > > > different one) would also need notification, even on x86, no? Since > > > > > it implicitly invalidates the previous mapping. > > > > > > > > > > I'm guessing the guest will avoid this by always unmapping before it > > > > > maps. We still need to consider this possibility when designing the > > > > > notifier interface though. > > > > > > > > > > It seems the real notification triggers here are: > > > > > * map - something is mapped which previously wasn't > > > > > * unmap - something is no longer mapped which was before > > > > > > > > > > Note that whether the second needs to be triggered depends on the > > > > > *previous* state of that IOBA range, *not* on the permissions of the > > > > > new mapping (if any). > > > > > > > > > > A "change" - replacing one mapping with another should count as both a > > > > > "map" and "unmap" event. > > > > > > > > Yeah... For MAP/UNMAP, it is strange in another way: e.g. for vhost, > > > > it doesn't care about map/unmap, it cares about invalidated cache. > > > > > > I think caring about invalidated cache *is* caring about unmap. It > > > doesn't matter whether the new mapping is something or nothing - if > > > the old mapping is no longer valid, you need to invalidate the cache, > > > yes? > > > > Yes, I think these two are exactly the same in implementation (vhost > > needs UNMAP events of course). So that's why I called it "a naming > > issue". :) > > > > > > > > > So > > > > IIUC this is a question about "naming" but not the implementations... > > > > I suppose it is really a matter of taste, and both work for me (either > > > > INVALIDATION/CHANGE or UNMAP/MAP). > > > > > > No.. it is a question of implementation. My point is that I don't > > > think the new permission is sufficient information to let you know if > > > a notification is necessary. You need to know if there was an > > > existing mapping at that IOBA. > > > > My understanding is that we don't need to know that. Because IIUC > > there are only map_page() and unmap_page() in guest IOMMU driver > > (please check dma_map_ops in kernel). There is no chance for anyone to > > "change" the content of the mapping, unless it calls unmap_page() then > > with a map_page(). In that case, we'll have two IOTLB invalidation > > requests. > > That's assuming a Linux guest using the current guest IOMMU model. > > I don't think we do so in practice, but the PAPR hypercall interface > allows in-place changing of a mapping. The interface is just "set > this IOPTE to this value". I see. Even if so, QEMU IOMMU emulation codes can convert one CHANGE request into UNMAP and a continuous MAP, right? Thanks, -- peterx