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

[Alex Williamson]

On Wed, 20 Nov 2019 10:35:03 -0800
Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx> wrote:

> On Tue, 19 Nov 2019 15:56:20 -0800
> "Alex Williamson" <alex.williamson@xxxxxxxxxx> wrote:
> 
> > On Mon, 11 Nov 2019 16:45:07 +0800
> > lantianyu1986@xxxxxxxxx wrote:
> >   
> > > From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
> > > 
> > > 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,

Alex