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

[Stuart Yoder]

On Tue, Apr 2, 2013 at 5:50 PM, Scott Wood <address@hidden> wrote:
> On 04/02/2013 04:38:45 PM, Alex Williamson wrote:
>>
>> On Tue, 2013-04-02 at 16:08 -0500, Stuart Yoder wrote:
>> > On Tue, Apr 2, 2013 at 3:57 PM, Scott Wood <address@hidden>
>> > wrote:
>> > >> >    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.
>> > >
>> > >
>> > > There's going to be special stuff no matter what.  This would keep it
>> > > separated from the IOMMU map code.
>> > >
>> > > I'm not sure what you mean by "overhead" here... the runtime overhead
>> > > of
>> > > setting things up is not particularly relevant as long as it's
>> > > reasonable.
>> > > If you mean development and maintenance effort, keeping things well
>> > > separated should help.
>> >
>> > We don't need to change DMA_MAP.  If we can simply add a new "type 2"
>> > ioctl that allows user space to set which windows are MSIs, it seems
>> > vastly
>> > less complex than an ioctl to supply a new fd, mmap of it, etc.
>> >
>> > So maybe 2 ioctls:
>> >     VFIO_IOMMU_GET_MSI_COUNT
>
>
> Do you mean a count of actual MSIs or a count of MSI banks used by the whole
> VFIO group?

I meant # of MSI banks, so VFIO_IOMMU_GET_MSI_BANK_COUNT would be better.

>> >     VFIO_IOMMU_MAP_MSI(iova, size)
>
>
> Not sure how you mean "size" to be used -- for MPIC it would be 4K per bank,
> and you can only map one bank at a time (which bank you're mapping should be
> a parameter, if only so that the kernel doesn't have to keep iteration state
> for you).

The intent was for user space to tell the kernel which windows to use
for MSI.   So I envisioned a total size of window-size * msi-bank-count.

Stuart