Re: NS83820 2.6.0-test5 driver seems unstable on IA64

From: Grant Grundler <iod00d_at_hp.com>
Date: 2003-09-24 06:38:19
On Tue, Sep 23, 2003 at 11:51:22AM -0700, David S. Miller wrote:
> > Even x86 pays at least a one cycle penalty for every misaligned access.
> 
> Yes, one cycle, and it's completely lost in the noise when it happens.

Depends on the app - for the networking stack, I agree.

To revisit Ben's comment: if we know something is likely to be misaligned,
a RISC processor can efficiently load both parts and merge them (one cycle
penalty vs a regular aligned load). Given misaligned accesses are infrequent
enough to affect performance, it makes sense to do this in SW because
it reduces cost of the HW design/test/mfg cycles.

...
> It is an unavoidable axoim in the kernel networking.  Unaligned accesses
> will happen, and they aren't a bug and therefore not worthy of mention
> in the kernel logs any more than "page was freed" :-)

Ok. If the kernel networking stack used get_unaligned() in the one place
Peter originally found, x86/sparc64?/et al wouldn't see a difference.
It would avoid traps on ia64 and parisc.  Bad idea?
Any other arches it might help/hurt on?

grant
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Received on Tue Sep 23 16:47:36 2003

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