Date: Wed, 18 Feb 2009 15:44:14 -0700 (MST) From: "M. Warner Losh" <imp@bsdimp.com> To: bms@incunabulum.net Cc: mips@FreeBSD.org, tinderbox@FreeBSD.org, current@FreeBSD.org Subject: Re: [head tinderbox] failure on mips/mips Message-ID: <20090218.154414.-1283372409.imp@bsdimp.com> In-Reply-To: <499C8396.2020000@incunabulum.net> References: <BE046876-F8EF-4269-9CD6-743483EC1869@mac.com> <20090218.120514.831271136.imp@bsdimp.com> <499C8396.2020000@incunabulum.net>
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In message: <499C8396.2020000@incunabulum.net>
Bruce Simpson <bms@incunabulum.net> writes:
: M. Warner Losh wrote:
: > ...
: > : The point being that programmers *do* code with certain
: > : assumptions and as soon as those assumptions don't hold on
: > : a platform, you end up worse off. My thoughts for MIPS n32
: > : are to make it behave like any "normal" 32-bit strong-
: > : alignment platform to avoid 1) a large number of runtime
: > : alignment faults -- which are a bigger performance bottleneck
: > : than forcing 64-bit integrals to be accessed with 2 32-bit
: > : accesses
: >
:
: FWIW, Linux use a type-length-value protocol for the netlink routing
: socket, so alignment issues of this kind are shifted around (forgive the
: pun).
FreeBSD does too for the most part, but it is only at the inter-message
level, not intra-message.
: > It also turns out that in this case, a simple (void *) is safe and
: > causes no issues because that time_t isn't accessed... It does give
: > one time to pause and think about it.
: >
:
: Yes, the void * cast works around the issue for now, but doesn't offer
: any guarantees that we are doing the right thing on strict alignment
: architectures.
: If the alignment *is* invalid, then we take the hit.
Right. The compiler doesn't check it, but I just did and things are
fine given the fields accessed.
But its that last bit that's the problem...
I also have changed the offending line in my tree to be u_long instead
of time_t since that is a better match to the protocol, and also gives
us 168 years of uptime before there's an issue.
Warner
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