Re: [PATCH] VFIO/VMBUS: Add VFIO VMBUS driver support

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 20 Nov 2019 12:07:15 -0700
Alex Williamson <alex.williamson@redhat.com> wrote:

> On Wed, 20 Nov 2019 10:35:03 -0800
> Stephen Hemminger <stephen@networkplumber.org> wrote:
> 
> > On Tue, 19 Nov 2019 15:56:20 -0800
> > "Alex Williamson" <alex.williamson@redhat.com> wrote:
> >   
> > > On Mon, 11 Nov 2019 16:45:07 +0800
> > > lantianyu1986@gmail.com wrote:
> > >     
> > > > From: Tianyu Lan <Tianyu.Lan@microsoft.com>
> > > > 
> > > > This patch is to add VFIO VMBUS driver support in order to expose
> > > > VMBUS devices to user space drivers(Reference Hyper-V UIO driver).
> > > > DPDK now has netvsc PMD driver support and it may get VMBUS resources
> > > > via VFIO interface with new driver support.
> > > > 
> > > > So far, Hyper-V doesn't provide virtual IOMMU support and so this
> > > > driver needs to be used with VFIO noiommu mode.      
> > > 
> > > Let's be clear here, vfio no-iommu mode taints the kernel and was a
> > > compromise that we can re-use vfio-pci in its entirety, so it had a
> > > high code reuse value for minimal code and maintenance investment.  It
> > > was certainly not intended to provoke new drivers that rely on this mode
> > > of operation.  In fact, no-iommu should be discouraged as it provides
> > > absolutely no isolation.  I'd therefore ask, why should this be in the
> > > kernel versus any other unsupportable out of tree driver?  It appears
> > > almost entirely self contained.  Thanks,
> > > 
> > > Alex    
> > 
> > The current VMBUS access from userspace is from uio_hv_generic
> > there is (and will not be) any out of tree driver for this.  
> 
> I'm talking about the driver proposed here.  It can only be used in a
> mode that taints the kernel that its running on, so why would we sign
> up to support 400 lines of code that has no safe way to use it?
>  
> > The new driver from Tianyu is to make VMBUS behave like PCI.
> > This simplifies the code for DPDK and other usermode device drivers
> > because it can use the same API's for VMBus as is done for PCI.  
> 
> But this doesn't re-use the vfio-pci API at all, it explicitly defines
> a new vfio-vmbus API over the vfio interfaces.  So a user mode driver
> might be able to reuse some vfio support, but I don't see how this has
> anything to do with PCI.
> 
> > Unfortunately, since Hyper-V does not support virtual IOMMU yet,
> > the only usage modle is with no-iommu taint.  
> 
> Which is what makes it unsupportable and prompts the question why it
> should be included in the mainline kernel as it introduces a
> maintenance burden and normalizes a usage model that's unsafe.  Thanks,

Many existing userspace drivers are unsafe:
  - out of tree DPDK igb_uio is unsafe.
  - VFIO with noiommu is unsafe.
  - hv_uio_generic is unsafe.

This new driver is not any better or worse. This sounds like a complete
repeat of the discussion that occurred before introducing VFIO noiommu mode.

Shouldn't vmbus vfio taint the kernel in the same way as vfio noiommu does?