在 2018年08月10日 星期五 09:12 下午, Jean-Philippe Brucker 写道:
Hi Kenneth,
On 10/08/18 04:39, Kenneth Lee wrote:
You can achieve everything you want to achieve with existing upstream
solution. Re-inventing a whole new driver infrastructure should really
be motivated with strong and obvious reasons.
I want to understand better of your idea. If I create some unified helper
APIs in drivers/iommu/, say:
wd_create_dev(parent_dev, wd_dev)
wd_release_dev(wd_dev)
The API create chrdev to take request from user space for open(resource
allocation), iomap, epoll (irq), and dma_map(with pasid automatically).
Do you think it is acceptable?
Maybe not drivers/iommu/ :) That subsystem only contains tools for
dealing with DMA, I don't think epoll, resource enumeration or iomap fit
in there.
Yes. I should consider where to put it carefully.
Creating new helpers seems to be precisely what we're trying to avoid in
this thread, and vfio-mdev does provide the components that you
describe, so I wouldn't discard it right away. When the GPU, net, block
or another subsystem doesn't fit your needs, either because your
accelerator provides some specialized function, or because for
performance reasons your client wants direct MMIO access, you can at
least build your driver and library on top of those existing VFIO
components:
* open allocates a partition of an accelerator.
* vfio_device_info, vfio_region_info and vfio_irq_info enumerates
available resources.
* vfio_irq_set deals with epoll.
* mmap gives you a private MMIO doorbell.
* vfio_iommu_type1 provides the DMA operations.
Currently missing:
* Sharing the parent IOMMU between mdev, which is also what the "IOMMU
aware mediated device" series tackles, and seems like a logical addition
to VFIO. I'd argue that the existing IOMMU ops (or ones implemented by
the SVA series) can be used to deal with this
* The interface to discover an accelerator near your memory node, or one
that you can chain with other devices. If I understood correctly the
conclusion was that the API (a topology description in sysfs?) should be
common to various subsystems, in which case vfio-mdev (or the mediating
driver) could also use it.
* The queue abstraction discussed on patch 3/7. Perhaps the current vfio
resource description of MMIO and IRQ is sufficient here as well, since
vendors tend to each implement their own queue schemes. If you need
additional features, read/write fops give the mediating driver a lot of
freedom. To support features that are too specific for drivers/vfio/ you
can implement a config space with capabilities and registers of your
choice. If you're versioning the capabilities, the code to handle them
could even be shared between different accelerator drivers and libraries.
Thank you, Jean,
The major reason that I want to remove dependency to VFIO is: I accepted
that the whole logic of VFIO was built on the idea of creating virtual
device.
Let's consider it in this way: We have hardware with IOMMU support. So
we create a default_domain to the particular IOMMU (unit) in the group
for the kernel driver to use it. Now the device is going to be used by a
VM or a Container. So we unbind it from the original driver, and put the
default_domain away, create a new domain for this particular use case.
So now the device shows up as a platform or pci device to the user
space. This is what VFIO try to provide. Mdev extends the scenario but
dose not change the intention. And I think that is why Alex emphasis
pre-allocating resource to the mdev.
But what WarpDrive need is to get service from the hardware itself and
set mapping to its current domain, aka defaut_domain. If we do it in
VFIO-mdev, it looks like the VFIO framework takes all the effort to put
the default_domain away and create a new one and be ready for user space
to use. But I tell him stop using the new domain and try the original one...
It is not reasonable, isn't it:)
So why don't I just take the request and set it into the default_domain
directly? The true requirement of WarpDrive is to let process set the
page table for particular pasid or substream id, so it can accept
command with address in the process space. It needs no device.
From this perspective, it seems there is no reason to keep it in VFIO.
Thanks
Kenneth
Thanks,
Jean
