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Mon, 06 Jan 2020 09:58:52 -0800 (PST) Date: Mon, 6 Jan 2020 12:58:50 -0500 From: Peter Xu To: Jean-Philippe Brucker Subject: Re: [PATCH for-5.0 v11 08/20] virtio-iommu: Implement translate Message-ID: <20200106175850.GC219677@xz-x1> References: <20191122182943.4656-9-eric.auger@redhat.com> <20191210193342.GJ3352@xz-x1> <44c0041d-68ad-796f-16cc-4bab7ba0f164@redhat.com> <20191219133308.GA4246@xz-x1> <9d58b293-ada0-353e-bba2-ad1f538dfc62@redhat.com> <20191219144936.GB50561@xz-x1> <9ec9d0d5-062b-f96b-c72c-4d15865ff9a1@redhat.com> <20191220162642.GA2626852@myrica> <20191220165100.GA3780@xz-x1> <20200106170634.GF2062@myrica> MIME-Version: 1.0 In-Reply-To: <20200106170634.GF2062@myrica> X-MC-Unique: 6TZneYgeOAiH1NTDvnEqxA-1 X-Mimecast-Spam-Score: 0 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: 205.139.110.61 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: yang.zhong@intel.com, peter.maydell@linaro.org, kevin.tian@intel.com, tnowicki@marvell.com, mst@redhat.com, jean-philippe.brucker@arm.com, quintela@redhat.com, qemu-devel@nongnu.org, armbru@redhat.com, Auger Eric , bharatb.linux@gmail.com, qemu-arm@nongnu.org, dgilbert@redhat.com, eric.auger.pro@gmail.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Jan 06, 2020 at 06:06:34PM +0100, Jean-Philippe Brucker wrote: > On Fri, Dec 20, 2019 at 11:51:00AM -0500, Peter Xu wrote: > > On Fri, Dec 20, 2019 at 05:26:42PM +0100, Jean-Philippe Brucker wrote: > > > There is at the virtio transport level: the driver sets status to > > > FEATURES_OK once it accepted the feature bits, and to DRIVER_OK once = its > > > fully operational. The virtio-iommu spec says: > > >=20 > > > If the driver does not accept the VIRTIO_IOMMU_F_BYPASS feature, th= e > > > device SHOULD NOT let endpoints access the guest-physical address s= pace. > > >=20 > > > So before features negotiation, there is no access. Afterwards it dep= ends > > > if the VIRTIO_IOMMU_F_BYPASS has been accepted by the driver. > >=20 > > Before enabling virtio-iommu device, should we still let the devices > > to access the whole system address space? I believe that's at least > > what Intel IOMMUs are doing. From code-wise, its: > >=20 > > if (likely(s->dmar_enabled)) { > > success =3D vtd_do_iommu_translate(vtd_as, vtd_as->bus, vtd_as-= >devfn, > > addr, flag & IOMMU_WO, &iotlb)= ; > > } else { > > /* DMAR disabled, passthrough, use 4k-page*/ > > iotlb.iova =3D addr & VTD_PAGE_MASK_4K; > > iotlb.translated_addr =3D addr & VTD_PAGE_MASK_4K; > > iotlb.addr_mask =3D ~VTD_PAGE_MASK_4K; > > iotlb.perm =3D IOMMU_RW; > > success =3D true; > > } > >=20 > > From hardware-wise, an IOMMU should be close to transparent if you > > never enable it, imho. >=20 > For hardware that's not necessarily the best choice. As cited in my > previous reply it has been shown to introduce vulnerabilities since > malicious devices can DMA during boot, before the OS takes control of the > IOMMU. The Arm SMMU allows an implementation to adopt a deny policy by > default. I see. But then how to read a sector from the block to at least boot an OS if we use a default-deny policy? Does it still need a mapping that is established somehow by someone before hand? >=20 > > Otherwise I'm confused on how a guest (with virtio-iommu) could boot > > with a normal BIOS that does not contain a virtio-iommu driver. For > > example, what if the BIOS needs to read some block sectors (as you > > mentioned)? >=20 > Ideally we should aim at supporting the device in both the BIOS and the > OS. Failing that, there should at least be a way to instantiate a > virtio-iommu device that is blocking by default, that cannot be bypassed > unless the controlling software decides to allow it. Could the bypass > policy be a command-line option to the virtio-iommu device? >=20 > [...] > > > Yes bypass mode assumes that devices and drivers aren't malicious, an= d the > > > IOMMU is only used for things like assigning devices to guest userspa= ce, > > > or having large contiguous DMA buffers. > >=20 > > Yes I agree. However again when the BYPASS flag was introduced, have > > you thought of introducing that flag per-device? IMHO that could be > > better because you have a finer granularity on controlling all these, > > so you'll be able to reject malicious devices but at the meantime > > grant permission to trusted devices. >=20 > At the moment that per-device behavior can be emulated by sending an > ATTACH request followed by identity MAP. It could be a little more > efficient to add a "bypass" flag to the ATTACH request and avoid setting > up the identity mapping manually, since the device then wouldn't need to > look up the mapping on translation, but I don't know how much it would > improve performance. The device could also cache the fact that the addres= s > space is identity-mapped, for the same result. The domain lookup has to > happen in any case, so you can never get the full iommu-free performance > with these mechanisms. IMHO it's really a matter of whether virtio-iommu wants to have a device layer besides the domain layer for the initial versions (just like VT-d has a device context, then it points to a domain, so it has these two layers). But I agree for the bypass feature it should work (though trying to detect "a device is put into an identital domain is bypassed" is still a bit tricky to me). And after all virtio-iommu is always extensible when needs come. Thanks, --=20 Peter Xu