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Wed, 8 Jan 2020 16:55:53 +0000 (UTC) Subject: Re: [PATCH for-5.0 v11 08/20] virtio-iommu: Implement translate To: Jean-Philippe Brucker , Peter Xu References: <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> <20200106175850.GC219677@xz-x1> <20200107101024.GB832497@myrica> From: Auger Eric Message-ID: Date: Wed, 8 Jan 2020 17:55:52 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <20200107101024.GB832497@myrica> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-MC-Unique: 0gi4Ex3TNY29WPq_H32XwQ-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 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, 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" Hi Jean-Philippe, Peter, On 1/7/20 11:10 AM, Jean-Philippe Brucker wrote: > On Mon, Jan 06, 2020 at 12:58:50PM -0500, Peter Xu wrote: >> 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: >>>>> >>>>> If the driver does not accept the VIRTIO_IOMMU_F_BYPASS feature, the >>>>> device SHOULD NOT let endpoints access the guest-physical address space. >>>>> >>>>> So before features negotiation, there is no access. Afterwards it depends >>>>> if the VIRTIO_IOMMU_F_BYPASS has been accepted by the driver. >>>> >>>> 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: >>>> >>>> if (likely(s->dmar_enabled)) { >>>> success = 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 = addr & VTD_PAGE_MASK_4K; >>>> iotlb.translated_addr = addr & VTD_PAGE_MASK_4K; >>>> iotlb.addr_mask = ~VTD_PAGE_MASK_4K; >>>> iotlb.perm = IOMMU_RW; >>>> success = true; >>>> } >>>> >>>> From hardware-wise, an IOMMU should be close to transparent if you >>>> never enable it, imho. >>> >>> 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? > > Yes, it looks like EDK II uses IOMMU operations in order to access those > devices on platforms where the IOMMU isn't default-bypass (AMD SEV support > is provided by edk2, and a VT-d driver seems provided by edk2-platforms). > However for OVMF we could just set the bypass feature bit in virtio-iommu > device, which doesn't even requires setting up the virtqueue. > > I'm missing a piece of the puzzle for Arm platforms though, because it > looks like Trusted Firmware-A sets up the default-deny policy on reset > even when it wasn't hardwired, but doesn't provide a service to create > SMMUv3 mappings for the bootloader. > > Thanks, > Jean > I think we have a concrete example for the above discussion. The AHCI. When running the virtio-iommu on x86 I get messages like: virtio_iommu_translate sid=250 is not known!! no buffer available in event queue to report event and a bunch of "AHCI: Failed to start FIS receive engine: bad FIS receive buffer address" messages (For each port) This was reported in my cover letter (*). This happens very early in the boot process, before the OS get the hand and before the virtio-iommu driver creates any mapping. It does not prevent the guest from booting though. Currently the virtio-iommu device checks the VIRTIO_IOMMU_F_BYPASS. If I overwrite it to true in the device, then, the guest boots without those messages. I share Peter's concern about having a different default policy than x86. Thanks Eric Note the migration issue reported in the cover letter is fixed now and was due to the migration priority unset.