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

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Message-ID: <501633EA.603@redhat.com>
Date:	Mon, 30 Jul 2012 09:12:42 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Boaz Harrosh <bharrosh@...asas.com>,
	Wang Sen <senwang@...ux.vnet.ibm.com>,
	linux-scsi@...r.kernel.org, JBottomley@...allels.com,
	stefanha@...ux.vnet.ibm.com, mc@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: virtio(-scsi) vs. chained sg_lists (was Re: [PATCH] scsi: virtio-scsi:
 Fix address translation failure of HighMem pages used by sg list)

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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