Re: Unaligned access on Alpha
On Sun, Feb 29, 2004 at 11:02:30AM -0600, Kelledin wrote: 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. 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? 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. :/ As of Linux 2.6, unaligned memory access is no longer handled for you by the kernel on alpha. These will instead generate SIGBUS errors, just as they have forever on Linux sparc. Any such bugs should be fixed, therefore. And in some cases, such as the evms case that currently came up on this list, such casts hide more serious bugs still. However, not all such warnings are actual bugs at all: you may be seeing the result of a pointer being down-cast and then up-cast again, which is a little sloppy (mostly from the standpoint that it causes compiler warnings), but shouldn't cause problems when running the app. Cheers, -- Steve Langasek postmodern programmer signature.asc Description: Digital signature
Re: Unaligned access on Alpha
Steve Langasek [EMAIL PROTECTED] writes: As of Linux 2.6, unaligned memory access is no longer handled for you by the kernel on alpha. These will instead generate SIGBUS errors, just as they have forever on Linux sparc. Are you sure? This was not the case with 2.6.0-test3, and the code still seems to be there in 2.6.3 (traps.c/do_entUnaUser). -- Falk
Re: Unaligned access on Alpha
Kelledin [EMAIL PROTECTED] 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
Re: Unaligned access on Alpha
On Sun, Feb 29, 2004 at 07:38:16PM +0100, Falk Hueffner wrote: Steve Langasek [EMAIL PROTECTED] writes: As of Linux 2.6, unaligned memory access is no longer handled for you by the kernel on alpha. These will instead generate SIGBUS errors, just as they have forever on Linux sparc. Are you sure? This was not the case with 2.6.0-test3, and the code still seems to be there in 2.6.3 (traps.c/do_entUnaUser). I'm not sure, as I've yet to run 2.6 on my alphas, but I have seen reports of SIGBUS on alpha under 2.6, which has definitely never happened before. (Maybe this behavior is now optional?) -- Steve Langasek postmodern programmer signature.asc Description: Digital signature
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 [EMAIL PROTECTED] 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
Re: Unaligned access on Alpha
On Sunday 29 February 2004 01:37 pm, Juraj Holtak wrote: 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) Hmmm...I take it that an unaligned access in kernel-land causes a crash/oops? In that case, I would very much like a copy of your patch! TIA, -- Kelledin If a server crashes in a server farm and no one pings it, does it still cost four figures to fix?
Re: Unaligned access on Alpha
Kelledin [EMAIL PROTECTED] writes: Hmmm...I take it that an unaligned access in kernel-land causes a crash/oops? No, it gets fixed up, too. -- Falk
