linux-kernel - Re: kmap_atomic_pfn for PCI BAR access?

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Date:	Mon, 7 Jul 2008 16:53:54 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Dave Airlie <airlied@...il.com>, Keith Packard <keithp@...thp.com>,
	Dave Airlie <airlied@...ux.ie>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: kmap_atomic_pfn for PCI BAR access?

On Thursday 26 June 2008 13:11, Jeremy Fitzhardinge wrote:
> Dave Airlie wrote:

> > doing tlb flush for iounmap is slow as all hell if you do it a lot,
> > and we can't afford to mmap the whole aperture it can 1GB.
>
> Maybe Nick's vmap reimplementation would help here.  It effectively
> allows you to map stuff into the vmalloc space, and do lazy tlb flushes
> to mitigate the cost of map/unmap.  He posted the patches week or so ago.

Yeah, it can _really_ help. I'd posted some performance numbers with
the patch which might prompt you to take another look at ioremap.

One thing I still haven't implemented in that patch are CPU-local
mappings (which only require a local flush to flush)... I found that
after the improvements I did implement, they didn't help much for
my workloads, so I suspect you might find the same thing... But anyway
if you really need the per-CPU mappings, it should be possible to
implement rather generically in vmap layer.

I would be very interested to know what sort of results you see with it
(compared to kmap_atomic and compared to vanilla ioremap)

Thanks,
Nick
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