Subject: Re: Generic IOMMU pooled allocator

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 24 Mar 2015 09:25:49 +1100

On Mon, 2015-03-23 at 12:54 -0400, Sowmini Varadhan wrote:

> If it was only an optimization (i.e., removing it would not break
> any functionality), and if this was done for older hardware,
> and *if* we believe that the direction of most architectures is to 
> follow the sun4v/HV model, then, given that the sun4u code only uses 1 
> arena pool anyway, one thought that I have for refactoring this
> is the following:
> 
> - Caller of iommu_tbl_range_alloc() can do the flush_all if they 
>   see start <= end for the one single pool 
> - lose the other ->flush_all invocation (i.e., the one that is
>   done when iommu_area_alloc() fails for pass == 0, and we reset
>   start to 0 to roll-back)
> 
> that would avoid the need for any iommu_tbl_ops in my patch-set.

You must hold the lock until you do the flush, otherwise somebody
else might allocate the not-yet-flushed areas and try to use them...
kaboom. However if that's the only callback left, pass it as an
argument.

> But it would imply that you would still take the perf hit for the roll-back
> if we failed the pass == 0 iteration through iommu_area_alloc().
> Perhaps this is an acceptable compromise in favor of cleaner code
> (again, assuming that current/future archs will all follow the HV
> based design).
> 
> --Sowmini
>