Re: "Unaligned access" on Alpha
many (i can say "all" for me) kernel unaligned accesses happen in the
netfilter code. so if u r running a firewall of any kind on EV56 u have a
real problem.
i wrote a small stupid (but working perfectly) patch that eliminates them to
zero.
if interessted just ask and i will post it (again)
Linux alpha 2.4.23-grsec #2 Tue Dec 2 22:05:21 CET 2003 alpha unknown
20:54:17 up 7 days, 18:38, 8 users, load average: 1.00, 1.00, 1.00
(dont care about the load the box is used for searching for big primes and the
load is just OK)
cpu : Alpha
cpu model : EV56
cpu variation : 0
cpu revision : 0
cpu serial number : Linux_is_Great!
system type : Miata
system variation : 0
system revision : 0
system serial number : MILO-2.2-17
cycle frequency [Hz] : 433061852 est.
timer frequency [Hz] : 1024.00
page size [bytes] : 8192
phys. address bits : 40
max. addr. space # : 127
BogoMIPS : 860.60
kernel unaligned acc : 0 (pc=0,va=0)
user unaligned acc : 27 (pc=120305e88,va=1204ceae4)
platform string : N/A
cpus detected : 0
the box is also my internet gateway and runs NAT and statefull packet filter
as u can see
no kernel un acc`s at all
some in userspace, but thats ok
the box is rock stable even under nonstop 100% load
(i had to reboot because of power failure a week ago)
the box gets hot as hell!
cheers,
Juraj Holtak
On Sunday 29 February 2004 18:10, Falk Hueffner wrote:
> Kelledin <kelledin+DAXP@skarpsey.dyndns.org> writes:
> > Lately I've been looking closely at the compile logs on my EV56
> > box, and I've noticed a particular gcc warning occurring
> > frequently:
> >
> > cast increases required alignment of target type.
> >
> > This worries me almost as much as casts between integers and
> > pointers of different size, but...even with all the warnings, I
> > don't get too many crashes.
>
> Well, not surprising, since this will only lead to a problem if a) the
> pointer is actually dereferenced and b) the alignment is actually
> wrong. Often this cannot occur and the warning is bogus.
>
> > Still, I'm compelled to wonder about the effect of unaligned
> > accesses, i.e. how severe is an unaligned access in user-space? How
> > about in kernel-space? How does the system handle them?
>
> A trap to PALcode occurs and the firmware hands over to the operating
> system. The OS emulates the access and resumes the program. Takes
> probably about 100-200 cycles.
>
> > I have a pretty clear idea what it would take to fix that warning
> > condition, but it occurs so frequently that I'm not sure it would be
> > worth the time. :/
>
> I don't think so, unless you actually see unaligned accesses in the
> syslog.
>
> --
> Falk
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