Re: virtio(-scsi) vs. chained sg_lists (was Re: [PATCH] scsi: virtio-scsi: Fix address translation failure of HighMem pages used by sg list)

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

Il 30/07/2012 01:50, Rusty Russell ha scritto:
>> Also, being the first user of chained scatterlist doesn't exactly give
>> me warm fuzzies.
> 
> We're far from the first user: they've been in the kernel for well over
> 7 years.  They were introduced for the block layer, but they tended to
> ignore private uses of scatterlists like this one.

Yeah, but sg_chain has no users in drivers, only a private one in
lib/scatterlist.c.  The internal API could be changed to something else
and leave virtio-scsi screwed...

> Yes, we should do this.  But note that this means an iteration, so we
> might as well combine the loops :)

I'm really bad at posting pseudo-code, but you can count the number of
physically-contiguous entries at the beginning of the list only.  So if
everything is contiguous, you use a single non-indirect buffer and save
a kmalloc.  If you use indirect buffers, I suspect it's much less
effective to collapse physically-contiguous entries.  More elaborate
heuristics do need a loop, though.

Paolo
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