Re: [Qemu-devel] RFC: vfio API changes needed for powerpc

[Stuart Yoder]

On Tue, Apr 2, 2013 at 3:32 PM, Alex Williamson
<address@hidden> wrote:
>> 2.   MSI window mappings
>>
>>    The more problematic question is how to deal with MSIs.  We need to
>>    create mappings for up to 3 MSI banks that a device may need to target
>>    to generate interrupts.  The Linux MSI driver can allocate MSIs from
>>    the 3 banks any way it wants, and currently user space has no way of
>>    knowing which bank may be used for a given device.
>>
>>    There are 3 options we have discussed and would like your direction:
>>
>>    A.  Implicit mappings -- with this approach user space would not
>>        explicitly map MSIs.  User space would be required to set the
>>        geometry so that there are 3 unused windows (the last 3 windows)
>>        for MSIs, and it would be up to the kernel to create the mappings.
>>        This approach requires some specific semantics (leaving 3 windows)
>>        and it potentially gets a little weird-- when should the kernel
>>        actually create the MSI mappings?  When should they be unmapped?
>>        Some convention would need to be established.
>
> VFIO would have control of SET/GET_ATTR, right?  So we could reduce the
> number exposed to userspace on GET and transparently add MSI entries on
> SET.

The number of windows is always power of 2 (and max is 256).  And to reduce
PAMU cache pressure you want to use the fewest number of windows
you can.    So, I don't see practically how we could transparently
steal entries to
add the MSIs.     Either user space knows to leave empty windows for
MSIs and by convention the kernel knows which windows those are (as
in option #A) or explicitly tell the kernel which windows (as in option #B).

> On x86 the interrupt remapper handles this transparently when MSI
> is enabled and userspace never gets direct access to the device MSI
> address/data registers.  What kind of restrictions do you have around
> adding and removing windows while the aperture is enabled?

The windows can be enabled/disabled event while the aperture is
enabled (pretty sure)...

>>    B.  Explicit mapping using DMA map flags.  The idea is that a new
>>        flag to DMA map (VFIO_DMA_MAP_FLAG_MSI) would mean that
>>        a mapping is to be created for the supplied iova.  No vaddr
>>        is given though.  So in the above example there would be a
>>        a dma map at 0x10000000 for 24KB (and no vaddr).   It's
>>        up to the kernel to determine which bank gets mapped where.
>>        So, this option puts user space in control of which windows
>>        are used for MSIs and when MSIs are mapped/unmapped.   There
>>        would need to be some semantics as to how this is used-- it
>>        only makes sense
>
> This could also be done as another "type2" ioctl extension.  What's the
> value to userspace in determining which windows are used by which banks?
> It sounds like the case that there are X banks and if userspace wants to
> use MSI it needs to leave X windows available for that.  Is this just
> buying userspace a few more windows to allow them the choice between MSI
> or RAM?

Yes, it would potentially give user space the flexibility some more windows.
It also makes more explicit when the MSI mappings are created.  In option
#A the MSI mappings would probably get created at the time of the first
normal DMA map.

So, you're saying with this approach you'd rather see a new type 2
ioctl instead of adding new flags to DMA map, right?

>>    C.  Explicit mapping using normal DMA map.  The last idea is that
>>        we would introduce a new ioctl to give user-space an fd to
>>        the MSI bank, which could be mmapped.  The flow would be
>>        something like this:
>>           -for each group user space calls new ioctl VFIO_GROUP_GET_MSI_FD
>>           -user space mmaps the fd, getting a vaddr
>>           -user space does a normal DMA map for desired iova
>>        This approach makes everything explicit, but adds a new ioctl
>>        applicable most likely only to the PAMU (type2 iommu).
>
> And the DMA_MAP of that mmap then allows userspace to select the window
> used?  This one seems like a lot of overhead, adding a new ioctl, new
> fd, mmap, special mapping path, etc.  It would be less overhead to just
> add an ioctl to enable MSI, maybe letting userspace pick which windows
> get used, but I'm still not sure what the value is to userspace in
> exposing it.  Thanks,

Thanks,
Stuart