Date: Wed, 18 Jul 2001 20:25:21 +0200 (CEST) From: kuehl@lgk.de To: Terry Lambert <tlambert2@mindspring.com> Cc: hackers@FreeBSD.org, Matthew Jacob <mjacob@feral.com>, John Baldwin <jhb@FreeBSD.org> Subject: Re: x86 unaligned access followup. Message-ID: <XFMail.010718202521.kuehl@lgk.de> In-Reply-To: <3B55B71C.CA7704BD@mindspring.com>
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On 18-Jul-01 Terry Lambert wrote:
> John Baldwin wrote:
>> It's the AC bit in eflags.
>
> Note that this will not trap 64 bit unaligned accesses, only 32.
And only at pl 3...
> Also note that this will play hell with some of the recent
> copy avoidance changes made by Bill Paul to the ethernet
> drivers, to avoid the expense of copying the packet, with
> the knowledge that there would be an increased overhead in
> the resulting packet field unaligned accesses when decoding
> IP packets...
>
> A "shakedown cruise" could end up being very rough... you
> would effectively need to check an "unaligned access in
> kernel is OK" flag in many of these instances, and fall back
> to doing the copy when it was false.
...therefore - never mind.
Perhaps some app code may break. ;-)
Lars
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