Subject: Re: [RFC] vhost: introduce mdev based hardware vhost backend

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 20 Apr 2018 06:50:21 +0300

On Fri, Apr 20, 2018 at 11:28:07AM +0800, Tiwei Bie wrote:
> 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.

But it is a pci device after all, isn't it?
IOMMU drivers certainly support that ...

Another issue with this approach is that internal
kernel issues leak out to the interface.

> > 
> > 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".
> 
> It's pretty cool. We also have some similar ideas.
> Cunming will talk more about this.
> 
> Best regards,
> Tiwei Bie

An extra benefit to this could be that requests with PASID
undergo an extra level of translation.
We could use it to avoid the need for shadowing on intel.

Something like this:
- expose to guest a standard virtio device (no pasid support)
- back it by virtio device with pasid support on the host
  by attaching same pasid to all queues

now - guest will build 1 level of page tables

we build first level page tables for requests with pasid
and point the IOMMU to use the guest supplied page tables
for the second level of translation.

Now we do need to forward invalidations but we no
longer need to set the CM bit and shadow valid entries.

> > 
> > Then using IOMMU with VFIO to limit access through queue to corrent
> > ranges of memory.
> > 
> > 
> > -- 
> > MST