Re: [Qemu-devel] [virtio-dev] Re: [PATCH v1 6/6] vhost-user: add VFIO based accelerators support

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Feb 04, 2018 at 01:49:46PM -0800, Alexander Duyck wrote:
> On Thu, Jan 25, 2018 at 9:57 PM, Tiwei Bie <tiwei.bie@intel.com> wrote:
> > On Fri, Jan 26, 2018 at 11:41:27AM +0800, Jason Wang wrote:
> >> On 2018年01月26日 07:59, Michael S. Tsirkin wrote:
> >> > > The virtual IOMMU isn't supported by the accelerators for now.
> >> > > Because vhost-user currently lacks of an efficient way to share
> >> > > the IOMMU table in VM to vhost backend. That's why the software
> >> > > implementation of virtual IOMMU support in vhost-user backend
> >> > > can't support dynamic mapping well.
> >> > What exactly is meant by that? vIOMMU seems to work for people,
> >> > it's not that fast if you change mappings all the time,
> >> > but e.g. dpdk within guest doesn't.
> >>
> >> Yes, software implementation support dynamic mapping for sure. I think the
> >> point is, current vhost-user backend can not program hardware IOMMU. So it
> >> can not let hardware accelerator to cowork with software vIOMMU.
> >
> > Vhost-user backend can program hardware IOMMU. Currently
> > vhost-user backend (or more precisely the vDPA driver in
> > vhost-user backend) will use the memory table (delivered
> > by the VHOST_USER_SET_MEM_TABLE message) to program the
> > IOMMU via vfio, and that's why accelerators can use the
> > GPA (guest physical address) in descriptors directly.
> >
> > Theoretically, we can use the IOVA mapping info (delivered
> > by the VHOST_USER_IOTLB_MSG message) to program the IOMMU,
> > and accelerators will be able to use IOVA. But the problem
> > is that in vhost-user QEMU won't push all the IOVA mappings
> > to backend directly. Backend needs to ask for those info
> > when it meets a new IOVA. Such design and implementation
> > won't work well for dynamic mappings anyway and couldn't
> > be supported by hardware accelerators.
> >
> >> I think
> >> that's another call to implement the offloaded path inside qemu which has
> >> complete support for vIOMMU co-operated VFIO.
> >
> > Yes, that's exactly what we want. After revisiting the
> > last paragraph in the commit message, I found it's not
> > really accurate. The practicability of dynamic mappings
> > support is a common issue for QEMU. It also exists for
> > vfio (hw/vfio in QEMU). If QEMU needs to trap all the
> > map/unmap events, the data path performance couldn't be
> > high. If we want to thoroughly fix this issue especially
> > for vfio (hw/vfio in QEMU), we need to have the offload
> > path Jason mentioned in QEMU. And I think accelerators
> > could use it too.
> >
> > Best regards,
> > Tiwei Bie
> 
> I wonder if we couldn't look at coming up with an altered security
> model for the IOMMU drivers to address some of the performance issues
> seen with typical hardware IOMMU?
> 
> In the case of most network devices, we seem to be moving toward a
> model where the Rx pages are mapped for an extended period of time and
> see a fairly high rate of reuse. As such pages mapped as being
> writable or read/write by the device are left mapped for an extended
> period of time while Tx pages, which are read only, are often
> mapped/unmapped since they are coming from some other location in the
> kernel beyond the driver's control.
> 
> If we were to somehow come up with a model where the read-only(Tx)
> pages had access to a pre-allocated memory mapped address, and the
> read/write(descriptor rings), write-only(Rx) pages were provided with
> dynamic addresses we might be able to come up with a solution that
> would allow for fairly high network performance while at least
> protecting from memory corruption. The only issue it would open up is
> that the device would have the ability to read any/all memory on the
> guest. I was wondering about doing something like this with the vIOMMU
> with VFIO for the Intel NICs this way since an interface like igb,
> ixgbe, ixgbevf, i40e, or i40evf would probably show pretty good
> performance under such a model and as long as the writable pages were
> being tracked by the vIOMMU. It could even allow for live migration
> support if the vIOMMU provided the info needed for migratable/dirty
> page tracking and we held off on migrating any of the dynamically
> mapped pages until after they were either unmapped or an FLR reset the
> device.
> 
> Thanks.
> 
> - Alex



It might be a good idea to change the iommu instead - how about a
variant of strict in intel iommu which forces an IOTLB flush after
invalidating a writeable mapping but not a RO mapping?  Not sure what the
name would be - relaxed-ro?

This is probably easier than poking at the drivers and net core.

Keeping the RX pages mapped in the IOMMU was envisioned for XDP.
That might be a good place to start.

-- 
MST