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Tsirkin" To: Srujana Challa Cc: Jason Wang , "virtualization@lists.linux.dev" , "kvm@vger.kernel.org" , Vamsi Krishna Attunuru , Shijith Thotton , Nithin Kumar Dabilpuram , Jerin Jacob , "joro@8bytes.org" , "will@kernel.org" Subject: Re: [EXTERNAL] Re: [PATCH] vdpa: Add support for no-IOMMU mode Message-ID: <20240910015607-mutt-send-email-mst@kernel.org> References: <20240530101823.1210161-1-schalla@marvell.com> <20240717054547-mutt-send-email-mst@kernel.org> <20240722034957-mutt-send-email-mst@kernel.org> <20240723070326-mutt-send-email-mst@kernel.org> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, Aug 28, 2024 at 09:08:13AM +0000, Srujana Challa wrote: > > Subject: RE: [EXTERNAL] Re: [PATCH] vdpa: Add support for no-IOMMU mode > > > > > On Tue, Jul 23, 2024 at 07:10:52AM +0000, Srujana Challa wrote: > > > > > On Mon, Jul 22, 2024 at 03:22:22PM +0800, Jason Wang wrote: > > > > > > On Fri, Jul 19, 2024 at 11:40 PM Srujana Challa > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > On Thu, May 30, 2024 at 03:48:23PM +0530, Srujana Challa wrote: > > > > > > > > > This commit introduces support for an UNSAFE, no-IOMMU > > > > > > > > > mode in the vhost-vdpa driver. When enabled, this mode > > > > > > > > > provides no device isolation, no DMA translation, no host > > > > > > > > > kernel protection, and cannot be used for device > > > > > > > > > assignment to virtual machines. It requires RAWIO > > > > > > > > > permissions and will taint the > > > kernel. > > > > > > > > > This mode requires enabling the > > > > > > > > "enable_vhost_vdpa_unsafe_noiommu_mode" > > > > > > > > > option on the vhost-vdpa driver. This mode would be useful > > > > > > > > > to get better performance on specifice low end machines > > > > > > > > > and can be leveraged by embedded platforms where > > > > > > > > > applications run in controlled > > > > > environment. > > > > > > > > > > > > > > > > > > Signed-off-by: Srujana Challa > > > > > > > > > > > > > > > > Thought hard about that. > > > > > > > > I think given vfio supports this, we can do that too, and > > > > > > > > the extension is > > > > > small. > > > > > > > > > > > > > > > > However, it looks like setting this parameter will > > > > > > > > automatically change the behaviour for existing userspace > > > > > > > > when > > > > > IOMMU_DOMAIN_IDENTITY is set. > > > > Our initial thought was to support only for no-iommu case, in which > > > > domain > > > itself > > > > won't be exist. So, we can modify the code as below to check for only > > > presence of domain. > > > > I think, only handling of no-iommu case wouldn't effect the > > > > existing > > > userspace. > > > > + if ((!domain) && vhost_vdpa_noiommu && capable(CAP_SYS_RAWIO)) > > { > > > > > > I would prefer some explicit action. > > > Just not specifying a domain is something I'd like to keep reserved > > > for something of more wide usefulness. > > Can we introduce a new feature like VHOST_BACKEND_F_NOIOMMU in > > VHOST_VDPA_BACKEND_FEATURES? We can have below logic based on this > > feature bit negotiation. > > Thanks. > Michael, could you please confirm if adding a new feature to VHOST_VDPA_BACKEND_FEATURES > is an appropriate solution to support no-IOMMU for the vhost-vdpa backend? Yes. So the idea is to require both a module parameter, and a flag set by userspace, to make sure users do not mistakenly try to assign such devices to VMs. Thanks. > > > > > > > > > > > > > > > > > > > > > > I suggest a new domain type for use just for this purpose. > > > > > > > > > > > > I'm not sure I get this, we want to bypass IOMMU, so it doesn't > > > > > > even have a doman. > > > > > > > > > > yes, a fake one. or come up with some other flag that userspace will set. > > > > > > > > > > > > This way if host has > > > > > > > > an iommu, then the same kernel can run both VMs with > > > > > > > > isolation and unsafe embedded apps without. > > > > > > > Could you provide further details on this concept? What > > > > > > > criteria would determine the configuration of the new domain > > > > > > > type? Would this require a boot parameter similar to > > > > > > > IOMMU_DOMAIN_IDENTITY, such as > > > > > iommu.passthrough=1 or iommu.pt? > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > > > > --- > > > > > > > > > drivers/vhost/vdpa.c | 23 +++++++++++++++++++++++ > > > > > > > > > 1 file changed, 23 insertions(+) > > > > > > > > > > > > > > > > > > diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c > > > > > > > > > index bc4a51e4638b..d071c30125aa 100644 > > > > > > > > > --- a/drivers/vhost/vdpa.c > > > > > > > > > +++ b/drivers/vhost/vdpa.c > > > > > > > > > @@ -36,6 +36,11 @@ enum { > > > > > > > > > > > > > > > > > > #define VHOST_VDPA_IOTLB_BUCKETS 16 > > > > > > > > > > > > > > > > > > +bool vhost_vdpa_noiommu; > > > > > > > > > > > > +module_param_named(enable_vhost_vdpa_unsafe_noiommu_mode, > > > > > > > > > + vhost_vdpa_noiommu, bool, 0644); > > > > > > > > > > > > +MODULE_PARM_DESC(enable_vhost_vdpa_unsafe_noiommu_mode, > > > > > > > > "Enable > > > > > > > > > +UNSAFE, no-IOMMU mode. This mode provides no device > > > > > > > > > +isolation, no DMA translation, no host kernel protection, > > > > > > > > > +cannot be used for device assignment to virtual machines, > > > > > > > > > +requires RAWIO permissions, and will taint the kernel. > > > > > > > > > +If you do not know what this is > > > > > for, step away. > > > > > > > > > +(default: false)"); > > > > > > > > > + > > > > > > > > > struct vhost_vdpa_as { > > > > > > > > > struct hlist_node hash_link; > > > > > > > > > struct vhost_iotlb iotlb; @@ -60,6 +65,7 @@ struct > > > > > > > > > vhost_vdpa { > > > > > > > > > struct vdpa_iova_range range; > > > > > > > > > u32 batch_asid; > > > > > > > > > bool suspended; > > > > > > > > > + bool noiommu_en; > > > > > > > > > }; > > > > > > > > > > > > > > > > > > static DEFINE_IDA(vhost_vdpa_ida); @@ -887,6 +893,10 @@ > > > > > > > > > static void vhost_vdpa_general_unmap(struct vhost_vdpa *v, { > > > > > > > > > struct vdpa_device *vdpa = v->vdpa; > > > > > > > > > const struct vdpa_config_ops *ops = vdpa->config; > > > > > > > > > + > > > > > > > > > + if (v->noiommu_en) > > > > > > > > > + return; > > > > > > > > > + > > > > > > > > > if (ops->dma_map) { > > > > > > > > > ops->dma_unmap(vdpa, asid, map->start, map->size); > > > > > > > > > } else if (ops->set_map == NULL) { @@ -980,6 +990,9 @@ > > > > > > > > > static int vhost_vdpa_map(struct vhost_vdpa *v, > > > > > > > > struct vhost_iotlb *iotlb, > > > > > > > > > if (r) > > > > > > > > > return r; > > > > > > > > > > > > > > > > > > + if (v->noiommu_en) > > > > > > > > > + goto skip_map; > > > > > > > > > + > > > > > > > > > if (ops->dma_map) { > > > > > > > > > r = ops->dma_map(vdpa, asid, iova, size, pa, perm, opaque); > > > > > > > > > } else if (ops->set_map) { @@ -995,6 +1008,7 @@ static > > > > > > > > > int vhost_vdpa_map(struct vhost_vdpa *v, > > > > > > > > struct vhost_iotlb *iotlb, > > > > > > > > > return r; > > > > > > > > > } > > > > > > > > > > > > > > > > > > +skip_map: > > > > > > > > > if (!vdpa->use_va) > > > > > > > > > atomic64_add(PFN_DOWN(size), > > > > > > > > > &dev->mm->pinned_vm); > > > > > > > > > > > > > > > > > > @@ -1298,6 +1312,7 @@ static int > > > > > > > > > vhost_vdpa_alloc_domain(struct > > > > > > > > vhost_vdpa *v) > > > > > > > > > struct vdpa_device *vdpa = v->vdpa; > > > > > > > > > const struct vdpa_config_ops *ops = vdpa->config; > > > > > > > > > struct device *dma_dev = vdpa_get_dma_dev(vdpa); > > > > > > > > > + struct iommu_domain *domain; > > > > > > > > > const struct bus_type *bus; > > > > > > > > > int ret; > > > > > > > > > > > > > > > > > > @@ -1305,6 +1320,14 @@ static int > > > > > > > > > vhost_vdpa_alloc_domain(struct > > > > > > > > vhost_vdpa *v) > > > > > > > > > if (ops->set_map || ops->dma_map) > > > > > > > > > return 0; > > > > > > > > > > > > > > > > > > + domain = iommu_get_domain_for_dev(dma_dev); > > > > > > > > > + if ((!domain || domain->type == IOMMU_DOMAIN_IDENTITY) > > > && > > > > > > > > > + vhost_vdpa_noiommu && capable(CAP_SYS_RAWIO)) { > > > > > > > > > > > > > > > > So if userspace does not have CAP_SYS_RAWIO instead of > > > > > > > > failing with a permission error the functionality changes silently? > > > > > > > > That's confusing, I think. > > > > > > > Yes, you are correct. I will modify the code to return error > > > > > > > when vhost_vdpa_noiommu is set and CAP_SYS_RAWIO is not set. > > > > > > > > > > > > > > Thanks. > > > > > > > > > > > > > > > > > > > > > > > > > + add_taint(TAINT_USER, LOCKDEP_STILL_OK); > > > > > > > > > + dev_warn(&v->dev, "Adding kernel taint for > > > > > > > > > + noiommu on > > > > > > > > device\n"); > > > > > > > > > + v->noiommu_en = true; > > > > > > > > > + return 0; > > > > > > > > > + } > > > > > > > > > bus = dma_dev->bus; > > > > > > > > > if (!bus) > > > > > > > > > return -EFAULT; > > > > > > > > > -- > > > > > > > > > 2.25.1 > > > > > > > > > > > >