> -----Original Message----- > From: Bie, Tiwei > Sent: Friday, April 20, 2018 11:28 AM > To: Michael S. Tsirkin <mst@xxxxxxxxxx> > Cc: Jason Wang <jasowang@xxxxxxxxxx>; alex.williamson@xxxxxxxxxx; > ddutile@xxxxxxxxxx; Duyck, Alexander H <alexander.h.duyck@xxxxxxxxx>; > virtio-dev@xxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > kvm@xxxxxxxxxxxxxxx; virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx; > netdev@xxxxxxxxxxxxxxx; Daly, Dan <dan.daly@xxxxxxxxx>; Liang, Cunming > <cunming.liang@xxxxxxxxx>; Wang, Zhihong <zhihong.wang@xxxxxxxxx>; Tan, > Jianfeng <jianfeng.tan@xxxxxxxxx>; Wang, Xiao W <xiao.w.wang@xxxxxxxxx>; > Tian, Kevin <kevin.tian@xxxxxxxxx> > Subject: Re: [RFC] vhost: introduce mdev based hardware vhost backend > > On Thu, Apr 19, 2018 at 09:40:23PM +0300, Michael S. Tsirkin wrote: > > On Tue, Apr 10, 2018 at 03:25:45PM +0800, Jason Wang wrote: > > > > > > One problem is that, different virtio ring compatible devices > > > > > > may have different device interfaces. That is to say, we will > > > > > > need different drivers in QEMU. It could be troublesome. And > > > > > > that's what this patch trying to fix. The idea behind this > > > > > > patch is very simple: mdev is a standard way to emulate device > > > > > > in kernel. > > > > > So you just move the abstraction layer from qemu to kernel, and > > > > > you still need different drivers in kernel for different device > > > > > interfaces of accelerators. This looks even more complex than > > > > > leaving it in qemu. As you said, another idea is to implement > > > > > userspace vhost backend for accelerators which seems easier and > > > > > could co-work with other parts of qemu without inventing new type of > messages. > > > > I'm not quite sure. Do you think it's acceptable to add various > > > > vendor specific hardware drivers in QEMU? > > > > > > > > > > I don't object but we need to figure out the advantages of doing it > > > in qemu too. > > > > > > Thanks > > > > To be frank kernel is exactly where device drivers belong. DPDK did > > move them to userspace but that's merely a requirement for data path. > > *If* you can have them in kernel that is best: > > - update kernel and there's no need to rebuild userspace > > - apps can be written in any language no need to maintain multiple > > libraries or add wrappers > > - security concerns are much smaller (ok people are trying to > > raise the bar with IOMMUs and such, but it's already pretty > > good even without) > > > > The biggest issue is that you let userspace poke at the device which > > is also allowed by the IOMMU to poke at kernel memory (needed for > > kernel driver to work). > > I think the device won't and shouldn't be allowed to poke at kernel memory. Its > kernel driver needs some kernel memory to work. But the device doesn't have > the access to them. Instead, the device only has the access to: > > (1) the entire memory of the VM (if vIOMMU isn't used) or > (2) the memory belongs to the guest virtio device (if > vIOMMU is being used). > > Below is the reason: > > For the first case, we should program the IOMMU for the hardware device based > on the info in the memory table which is the entire memory of the VM. > > For the second case, we should program the IOMMU for the hardware device > based on the info in the shadow page table of the vIOMMU. > > So the memory can be accessed by the device is limited, it should be safe > especially for the second case. > > My concern is that, in this RFC, we don't program the IOMMU for the mdev > device in the userspace via the VFIO API directly. Instead, we pass the memory > table to the kernel driver via the mdev device (BAR0) and ask the driver to do the > IOMMU programming. Someone may don't like it. The main reason why we don't > program IOMMU via VFIO API in userspace directly is that, currently IOMMU > drivers don't support mdev bus. > > > > > Yes, maybe if device is not buggy it's all fine, but it's better if we > > do not have to trust the device otherwise the security picture becomes > > more murky. > > > > I suggested attaching a PASID to (some) queues - see my old post > > "using PASIDs to enable a safe variant of direct ring access". > Ideally we can have a device binding with normal driver in host, meanwhile support to allocate a few queues attaching with PASID on-demand. By vhost mdev transport channel, the data path ability of queues(as a device) can expose to qemu vhost adaptor as a vDPA instance. Then we can avoid VF number limitation, providing vhost data path acceleration in a small granularity. > It's pretty cool. We also have some similar ideas. > Cunming will talk more about this. > > Best regards, > Tiwei Bie > > > > > Then using IOMMU with VFIO to limit access through queue to corrent > > ranges of memory. > > > > > > -- > > MST
