Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

[Alex Williamson <alex.williamson-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Wed, 26 May 2021 23:40:02 +0530
Kirti Wankhede <kwankhede-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org> wrote:

> On 5/26/2021 4:22 AM, Alex Williamson wrote:
> > On Wed, 26 May 2021 00:56:30 +0530
> > Kirti Wankhede <kwankhede-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org> wrote:
> >   
> >> On 5/25/2021 5:07 AM, Jason Gunthorpe wrote:  
> >>> On Mon, May 24, 2021 at 05:52:58PM +1000, David Gibson wrote:
> >>>      
> >>>>>> I don't really see a semantic distinction between "always one-device
> >>>>>> groups" and "groups don't matter".  Really the only way you can afford
> >>>>>> to not care about groups is if they're singletons.  
> >>>>>
> >>>>> The kernel driver under the mdev may not be in an "always one-device"
> >>>>> group.  
> >>>>
> >>>> I don't really understand what you mean by that.  
> >>>
> >>> I mean the group of the mdev's actual DMA device may have multiple
> >>> things in it.
> >>>        
> >>>>> It is a kernel driver so the only thing we know and care about is that
> >>>>> all devices in the HW group are bound to kernel drivers.
> >>>>>
> >>>>> The vfio device that spawns from this kernel driver is really a
> >>>>> "groups don't matter" vfio device because at the IOMMU layer it should
> >>>>> be riding on the physical group of the kernel driver.  At the VFIO
> >>>>> layer we no longer care about the group abstraction because the system
> >>>>> guarentees isolation in some other way.  
> >>>>
> >>>> Uh.. I don't really know how mdevs are isolated from each other.  I
> >>>> thought it was because the physical device providing the mdevs
> >>>> effectively had an internal IOMMU (or at least DMA permissioning) to
> >>>> isolate the mdevs, even though the physical device may not be fully
> >>>> isolated.
> >>>>
> >>>> In that case the virtual mdev is effectively in a singleton group,
> >>>> which is different from the group of its parent device.  
> >>>      
> >>
> >> That's correct.
> >>  
> >>> That is one way to view it, but it means creating a whole group
> >>> infrastructure and abusing the IOMMU stack just to create this
> >>> nonsense fiction.  
> >>
> >> I really didn't get how this abuse the IOMMU stack.
> >> mdev can be used in 3 different ways:
> >> 1. non-iommu backed mdev devices where mdev vendor driver takes care to
> >> DMA map (iommu_map) and isolation is through device hardware internal
> >> MMU. Here vfio_iommu_type1 module provides a way to validate and pin
> >> pages required by mdev device for DMA mapping. Then IOMMU mapping is
> >> done by mdev vendor driver which is owner driver of physical device.
> >>
> >> 2. iommu backed mdev devices for SRIOV where mdev device is created per
> >> VF (mdev device == VF device) then that mdev device has same iommu
> >> protection scope as VF associated to it. Here mdev device is virtual
> >> device which uses features of mdev and represents underlying VF device,
> >> same as vfio-pci but with additional mdev features.  
> > 
> > What features would those be?  There are no mdev specific parts of the
> > vfio uAPI.
> > 
> > The mdev device is a virtual device, by why it it virtual in this case?
> > Aren't we effectively assigning the VF itself (mdev device == VF device)
> > with a bunch of extra support code to fill in the gaps of the VF
> > implementing the complete device model in hardware?
> > 
> > We're effectively creating this virtual device, creating a fake IOMMU
> > group, and trying to create this association of this virtual device to
> > the real VF in order to shoehorn it into the mdev model.  What do we
> > get from that model other than lifecycle management (ie. type selection)
> > and re-use of a bunch of code from the driver supporting the 1) model
> > above?
> >   
> 
> Yes, the lifecycle management which is in mdev is not in vfio-pci variant.
> 
> > This specific model seems better served by a device specific peer
> > driver to vfio-pci (ie. a "vfio-pci variant").  You effectively already
> > have the code for this driver, it's just in the format of an mdev
> > driver rather than a vfio "bus driver".  The work Jason references
> > relative to Max aims to make these kinds of drivers easier to implement
> > through re-use of vfio-pci code.
> > 
> > There are certainly other solutions we could come up with for selecting
> > a specific device type for a vfio-pci variant driver to implement other
> > than pretending this model actually belongs in mdev, right?  Thanks,
> >   
> 
> Sure and would like to see type selection mechanism to be implemented in 
> vfio-pci variant.


A driver provided sysfs attribute would obviously fill the short
term gap, long term maybe this would be standardized via netlink.  It
seems a bit analogous to setting the MAC address for a VF on an SR-IOV
NIC or VF namespace configuration for an SR-IOV NVMe device.  Thanks,

Alex