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Tue, 17 Mar 2020 07:13:18 -0700 (PDT) Date: Tue, 17 Mar 2020 10:13:16 -0400 From: Peter Xu To: Jason Wang Subject: Re: [PATCH V2] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM Message-ID: <20200317141316.GA199571@xz-x1> References: <20200226163618.31aa86ed.pasic@linux.ibm.com> <20200226115009-mutt-send-email-mst@kernel.org> <20200227140215.2d12149c.pasic@linux.ibm.com> <20200227104233-mutt-send-email-mst@kernel.org> <20200313134446.782c5f7c.pasic@linux.ibm.com> <20200313112902-mutt-send-email-mst@kernel.org> <20200313163122.GB95517@xz-x1> <20200316131723-mutt-send-email-mst@kernel.org> <20200316181405.GB184827@xz-x1> <158fbd3b-1285-e18d-b535-a1e7a9802f1f@redhat.com> MIME-Version: 1.0 In-Reply-To: <158fbd3b-1285-e18d-b535-a1e7a9802f1f@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 63.128.21.74 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Tom Lendacky , "Singh, Brijesh" , "Michael S. Tsirkin" , qemu-stable@nongnu.org, qemu-devel@nongnu.org, Halil Pasic , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, Mar 17, 2020 at 11:04:26AM +0800, Jason Wang wrote: >=20 > On 2020/3/17 =E4=B8=8A=E5=8D=882:14, Peter Xu wrote: > > On Mon, Mar 16, 2020 at 01:19:54PM -0400, Michael S. Tsirkin wrote: > > > On Fri, Mar 13, 2020 at 12:31:22PM -0400, Peter Xu wrote: > > > > On Fri, Mar 13, 2020 at 11:29:59AM -0400, Michael S. Tsirkin wrote: > > > > > On Fri, Mar 13, 2020 at 01:44:46PM +0100, Halil Pasic wrote: > > > > > > [..] > > > > > > > > CCing Tom. @Tom does vhost-vsock work for you with SEV and = current qemu? > > > > > > > >=20 > > > > > > > > Also, one can specify iommu_platform=3Don on a device that = ain't a part of > > > > > > > > a secure-capable VM, just for the fun of it. And that break= s > > > > > > > > vhost-vsock. Or is setting iommu_platform=3Don only valid i= f > > > > > > > > qemu-system-s390x is protected virtualization capable? > > > > > > > >=20 > > > > > > > > BTW, I don't have a strong opinion on the fixes tag. We cur= rently do not > > > > > > > > recommend setting iommu_platform, and thus I don't think we= care too > > > > > > > > much about past qemus having problems with it. > > > > > > > >=20 > > > > > > > > Regards, > > > > > > > > Halil > > > > > > > Let's just say if we do have a Fixes: tag we want to set it c= orrectly to > > > > > > > the commit that needs this fix. > > > > > > >=20 > > > > > > I finally did some digging regarding the performance degradatio= n. For > > > > > > s390x the performance degradation on vhost-net was introduced b= y commit > > > > > > 076a93d797 ("exec: simplify address_space_get_iotlb_entry"). Be= fore > > > > > > IOMMUTLBEntry.addr_mask used to be based on plen, which in turn= was > > > > > > calculated as the rest of the memory regions size (from address= ), and > > > > > > covered most of the guest address space. That is we didn't have= a whole > > > > > > lot of IOTLB API overhead. > > > > > >=20 > > > > > > With commit 076a93d797 I see IOMMUTLBEntry.addr_mask =3D=3D 0xf= ff which comes > > > > > > as ~TARGET_PAGE_MASK from flatview_do_translate(). To have thin= gs working > > > > > > properly I applied 75e5b70e6, b021d1c044, and d542800d1e on the= level of > > > > > > 076a93d797 and 076a93d797~1. > > > > > Peter, what's your take on this one? > > > > Commit 076a93d797 was one of the patchset where we want to provide > > > > sensible IOTLB entries and also that should start to work with huge > > > > pages. > > > So the issue bundamentally is that it > > > never produces entries larger than page size. > > >=20 > > > Wasteful even just with huge pages, all the more > > > so which passthrough which could have giga-byte > > > entries. > > >=20 > > > Want to try fixing that? > > Yes we can fix that, but I'm still not sure whether changing the > > interface of address_space_get_iotlb_entry() to cover adhoc regions is > > a good idea, because I think it's still a memory core API and imho it > > would still be good to have IOTLBs returned to be what the hardware > > will be using (always page aligned IOTLBs). Also it would still be > > not ideal because vhost backend will still need to send the MISSING > > messages and block for each of the continuous guest memory ranges > > registered, so there will still be misterious delay. Not to say > > logically all the caches can be invalidated too so in that sense I > > think it's as hacky as the vhost speedup patch mentioned below.. > >=20 > > Ideally I think vhost should be able to know when PT is enabled or > > disabled for the device, so the vhost backend (kernel or userspace) > > should be able to directly use GPA for DMA. That might need some new > > vhost interface. >=20 >=20 > Yes but I think we don't need another API since we can send GPA->HVA mapp= ing > via device IOTLB API when we find there's no DMA translation at all (eith= er > PT or no vIOMMU). Jason, Do you mean what we've worked on before? https://lists.gnu.org/archive/html/qemu-devel/2017-06/msg00574.html (I just read the previous discussion on that patch, it seems to be exactly what we've discussed again...) Thanks, >=20 > Vhost doesn't need to know whether an address is an IOVA (vIOMMU) , GPA (= no > vIOMMU), or even HVA (dpdk virtio-user). >=20 > Thanks >=20 >=20 > >=20 > > For the s390's specific issue, I would think Jason's patch an simple > > and ideal solution already. > >=20 > > Thanks, > >=20 >=20 --=20 Peter Xu