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[79.180.7.202]) by smtp.gmail.com with ESMTPSA id f203sm436818wmf.18.2020.03.16.10.19.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 16 Mar 2020 10:19:58 -0700 (PDT) Date: Mon, 16 Mar 2020 13:19:54 -0400 From: "Michael S. Tsirkin" To: Peter Xu Subject: Re: [PATCH V2] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM Message-ID: <20200316131723-mutt-send-email-mst@kernel.org> References: <20200226094357.25061-1-jasowang@redhat.com> <20200226142839.4263de9b.pasic@linux.ibm.com> <20200226083654-mutt-send-email-mst@kernel.org> <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> MIME-Version: 1.0 In-Reply-To: <20200313163122.GB95517@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 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: > > > [..] > > > > >=20 > > > > > CCing Tom. @Tom does vhost-vsock work for you with SEV and curren= t 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 breaks > > > > > 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 currently= 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 correct= ly to > > > > the commit that needs this fix. > > > >=20 > > >=20 > > > I finally did some digging regarding the performance degradation. For > > > s390x the performance degradation on vhost-net was introduced by comm= it > > > 076a93d797 ("exec: simplify address_space_get_iotlb_entry"). Before > > > 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 who= le > > > lot of IOTLB API overhead. > > >=20 > > > With commit 076a93d797 I see IOMMUTLBEntry.addr_mask =3D=3D 0xfff whi= ch comes > > > as ~TARGET_PAGE_MASK from flatview_do_translate(). To have things wor= king > > > properly I applied 75e5b70e6, b021d1c044, and d542800d1e on the 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 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. Wasteful even just with huge pages, all the more so which passthrough which could have giga-byte entries. Want to try fixing that? > 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 merged: >=20 > https://lists.gnu.org/archive/html/qemu-devel/2017-06/msg00574.html >=20 > Regarding to the current patch, I'm not sure I understand it > correctly, but is that performance issue only happens when (1) there's > no intel-iommu device, and (2) there is iommu_platform=3Don specified > 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 - when we > want to force IOMMU to be enabled from host side, and it should always > be used with a vIOMMU device. >=20 > However I also agree that from performance POV this patch helps for > this quite special case. >=20 > Thanks, >=20 > --=20 > Peter Xu