Re: Intel IOMMU (and IOMMU for Virtualization) performances

[Grant Grundler]

On Fri, Jun 6, 2008 at 1:21 PM, Muli Ben-Yehuda <muli@xxxxxxxxxx> wrote:
....
>> It's possible to split up one flat address space and share the IOMMU
>> among several users. Each user gets her own segment of bitmap and
>> corresponding IO Pdir. So I don't see allocation policy as a strong
>> reason to use Red/Black Tree.
>
> Do you mean multiple users sharing the same I/O address space (but
> each user using a different segment), or multiple users, each with its
> own I/O address space, but only using a specific segment of that
> address space and using a single bitmap to represent free space in all
> segments?

Yes, I meant the former.

> If the former, then you are losing some of the benefit of
> the IOMMU since all users can DMA to other users areas (same I/O
> address space). If the latter, having a bitmap per IO address space
> seems simpler and would have the same memory consumption.


Agreed. It's a trade off.

...
>> I've never been able to come up with a good heuristic for
>> determining the size of the IOVA space. It generally does NOT need
>> to map all of Host Physical RAM.  The actual requirement depends
>> entirely on the workload, type and number of IO devices
>> installed. The problem is we don't know any of those things until
>> well after the IOMMU is already needed.
>
> Why not do what hash-tables implementation do, start small and resize
> when we approach half-full?

Historically the IOMMUs needed physically contiguous memory and
resizing essentially meant quiescing all DMA, moving the IO Pdir data
to the new bigger location,  allocating a new bitmap and cloning the
state into that as well, and then resuming DMA operations. The DMA
quiesce requirement effectively meant a reboot. My understanding of
Vt-d is the ranges can be added range at a time and thus can be easily
resized. But it will mean more complex logic in the IOMMU bitmap
handling for a domain which owns multiple bitmaps and thus a slightly
higher CPU utilization cost. At least that's my guess. I'm not working
on any IOMMU code lately...

thanks,
grant
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