On Mon, 2015-03-23 at 19:19 -0400, Sowmini Varadhan wrote:

What I've tried to do is to have a bool large_pool arg passed
to iommu_tbl_pool_init. In my observation (instrumented for scsi, ixgbe), 
we never allocate more than 4 pages at a time, so I pass in 
large_pool == false for all the sparc platforms. 

But that might not be necessary. If indeed we very rarely use the large
pool, then just make it flush always. My feeling is that it will only
ever be used at driver init/remove time when allocating things like
descriptor rings, where the flush overhead dont' matter.

quoted

Or we can decide that large allocs are rare (typically
pci_alloc_consistent, ie, driver init time), and thus always flush on
them (or rather on free of a large chunk). David, what's your take
there ? I have a feeling that should work fine without a noticeable
performance issue...

I would also keep a "dirty" flag set on any free and cleared on any
flush to avoid more spurrious flushes, but here too the benefit might be
in the noise.

Ben.