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[79.183.62.79]) by smtp.gmail.com with ESMTPSA id l12sm2247377qtp.44.2020.03.17.07.55.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Mar 2020 07:55:23 -0700 (PDT) Date: Tue, 17 Mar 2020 10:55:18 -0400 From: "Michael S. Tsirkin" To: Peter Xu Subject: Re: [PATCH V2] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM Message-ID: <20200317105312-mutt-send-email-mst@kernel.org> References: <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> <20200317022707-mutt-send-email-mst@kernel.org> <20200317143904.GC199571@xz-x1> MIME-Version: 1.0 In-Reply-To: <20200317143904.GC199571@xz-x1> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii 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" , Jason Wang , qemu-devel@nongnu.org, qemu-stable@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 10:39:04AM -0400, Peter Xu wrote: > On Tue, Mar 17, 2020 at 02:28:42AM -0400, Michael S. Tsirkin wrote: > > On Mon, Mar 16, 2020 at 02:14:05PM -0400, 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 wrot= e: > > > > > > On Fri, Mar 13, 2020 at 01:44:46PM +0100, Halil Pasic wrote: > > > > > > > [..] > > > > > > > > >=20 > > > > > > > > > CCing Tom. @Tom does vhost-vsock work for you with SEV an= d current qemu? > > > > > > > > >=20 > > > > > > > > > Also, one can specify iommu_platform=3Don on a device tha= t ain't a part of > > > > > > > > > a secure-capable VM, just for the fun of it. And that bre= aks > > > > > > > > > vhost-vsock. Or is setting iommu_platform=3Don only valid= if > > > > > > > > > qemu-system-s390x is protected virtualization capable? > > > > > > > > >=20 > > > > > > > > > BTW, I don't have a strong opinion on the fixes tag. We c= urrently 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 > > > > > > > >=20 > > > > > > > >=20 > > > > > > > > Let's just say if we do have a Fixes: tag we want to set it= correctly to > > > > > > > > the commit that needs this fix. > > > > > > > >=20 > > > > > > >=20 > > > > > > > I finally did some digging regarding the performance degradat= ion. For > > > > > > > s390x the performance degradation on vhost-net was introduced= by commit > > > > > > > 076a93d797 ("exec: simplify address_space_get_iotlb_entry"). = Before > > > > > > > IOMMUTLBEntry.addr_mask used to be based on plen, which in tu= rn was > > > > > > > calculated as the rest of the memory regions size (from addre= ss), and > > > > > > > covered most of the guest address space. That is we didn't ha= ve a whole > > > > > > > lot of IOTLB API overhead. > > > > > > >=20 > > > > > > > With commit 076a93d797 I see IOMMUTLBEntry.addr_mask =3D=3D 0= xfff which comes > > > > > > > as ~TARGET_PAGE_MASK from flatview_do_translate(). To have th= ings working > > > > > > > properly I applied 75e5b70e6, b021d1c044, and d542800d1e on t= he level of > > > > > > > 076a93d797 and 076a93d797~1. > > > > > >=20 > > > > > > Peter, what's your take on this one? > > > > >=20 > > > > > Commit 076a93d797 was one of the patchset where we want to provid= e > > > > > sensible IOTLB entries and also that should start to work with hu= ge > > > > > pages. > > > >=20 > > > > 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? > > >=20 > > > 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 i= s > > > 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). > >=20 > > E.g. with virtio-iommu, there's no hardware in sight. > > Even with e.g. VTD page aligned does not mean TARGET_PAGE, > > can be much bigger. >=20 > Right. Sorry to be unclear, but I meant the emulated device (in this > case for x86 it's VT-d) should follow the hardware. Here the page > mask is decided by VT-d in vtd_iommu_translate() for PT mode which is > 4K only. For another example, ARM SMMU is doing similar thing (return > PAGE_SIZE when PT enabled, smmuv3_translate()). That actually makes > sense to me. On the other hand, I'm not sure whether there's side > effect if we change this to cover the whole address space for PT. >=20 > Thanks, Well we can translate a batch of entries in a loop, and as long as VA/PA mappings are consistent, treat the batch as one. This is a classical batching approach and not doing this is a classical reason for bad performance. > >=20 > > > 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 > > > For the s390's specific issue, I would think Jason's patch an simple > > > and ideal solution already. > > >=20 > > > Thanks, > > >=20 > > > >=20 > > > >=20 > > > > > Frankly speaking after a few years I forgot the original > > > > > motivation of that whole thing, but IIRC there's a patch that was > > > > > trying to speedup especially for vhost but I noticed it's not mer= ged: > > > > >=20 > > > > > https://lists.gnu.org/archive/html/qemu-devel/2017-06/msg00574.ht= ml > > > > >=20 > > > > > Regarding to the current patch, I'm not sure I understand it > > > > > correctly, but is that performance issue only happens when (1) th= ere's > > > > > no intel-iommu device, and (2) there is iommu_platform=3Don speci= fied > > > > > for the vhost backend? > > > > >=20 > > > > > If so, I'd confess I am not too surprised if this fails the boot = with > > > > > vhost-vsock because after all we speicified iommu_platform=3Don > > > > > explicitly in the cmdline, so if we want it to work we can simply > > > > > remove that iommu_platform=3Don when vhost-vsock doesn't support = it > > > > > yet... I thougth iommu_platform=3Don was added for that case - w= hen we > > > > > want to force IOMMU to be enabled from host side, and it should a= lways > > > > > be used with a vIOMMU device. > > > > >=20 > > > > > However I also agree that from performance POV this patch helps f= or > > > > > this quite special case. > > > > >=20 > > > > > Thanks, > > > > >=20 > > > > > --=20 > > > > > Peter Xu > > > >=20 > > >=20 > > > --=20 > > > Peter Xu > >=20 >=20 > --=20 > Peter Xu