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Mon, 16 Mar 2020 10:32:00 -0700 (PDT) Date: Mon, 16 Mar 2020 13:31:58 -0400 From: Peter Xu To: Halil Pasic Subject: Re: [PATCH V2] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM Message-ID: <20200316173158.GA184827@xz-x1> References: <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> <20200316175737.365d7b32.pasic@linux.ibm.com> MIME-Version: 1.0 In-Reply-To: <20200316175737.365d7b32.pasic@linux.ibm.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] [fuzzy] X-Received-From: 216.205.24.74 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" , Jason Wang , qemu-stable@nongnu.org, qemu-devel@nongnu.org, Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Mar 16, 2020 at 05:57:37PM +0100, Halil Pasic wrote: > On Fri, 13 Mar 2020 12:31:22 -0400 > Peter Xu wrote: >=20 > > 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 curr= ent 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 current= ly do not > > > > > > recommend setting iommu_platform, and thus I don't think we car= e 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 corre= ctly to > > > > > the commit that needs this fix. > > > > >=20 > > > >=20 > > > > I finally did some digging regarding the performance degradation. F= or > > > > s390x the performance degradation on vhost-net was introduced by co= mmit > > > > 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), a= nd > > > > covered most of the guest address space. That is we didn't have a w= hole > > > > lot of IOTLB API overhead. > > > >=20 > > > > With commit 076a93d797 I see IOMMUTLBEntry.addr_mask =3D=3D 0xfff w= hich comes > > > > as ~TARGET_PAGE_MASK from flatview_do_translate(). To have things w= orking > > > > properly I applied 75e5b70e6, b021d1c044, and d542800d1e on the lev= el 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. 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 [1] > >=20 >=20 > From the looks of it, I don't think we would have seen that big > performance degradation had this patch been included. I can give > it a spin if you like. Shall I? >=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 >=20 > I can confirm, that your description covers my scenario. I didn't > investigate what happens when we have an intel-iommu, because s390 does > not do intel-iommu. I can also confirm that no performance degradation > is observed when the virtio-net has iommu_platform=3Doff. The property > iommu_platform is a virtio device (and not a backend) level property. > =20 >=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 >=20 > The problem is that the virtio feature bit F_ACCESS_PLATFORM, which is > directly controlled by the iommu_platform proprerty stands for two things > 1) need to do IOVA translation > 2) the access of the device to the guests RAM is restricted. >=20 > There are cases where 2) does apply and 1) does not. We need to specify > iommu_platform=3Don to make the virtio implementation in the guest use > the dma api, because we need to grant access to memory as required. But > we don't need translation and we don't have a vIOMMU. I see the point of this patch now. I'm still unclear on how s390 works for DMA protection, but it seems totally different from the IOMMU model on x86/arm. Considering this, please ignore above patch [1] because that's hackish in all cases to play with iotlb caches, and current patch should be much better (and easier) IMHO. Thanks, --=20 Peter Xu