Date: Tue, 27 Aug 2013 07:25:04 -0600 From: Warner Losh <imp@bsdimp.com> To: Michael Tuexen <tuexen@freebsd.org> Cc: freebsd-arm <freebsd-arm@FreeBSD.org>, Andre Oppermann <andre@FreeBSD.org> Subject: Re: ARM network trouble after recent mbuf changes Message-ID: <D198D7DD-DE15-4438-9931-F13B0F17ED9B@bsdimp.com> In-Reply-To: <627451FE-4F20-4E4D-9B24-59E0F340EF75@freebsd.org> References: <1377550636.1111.156.camel@revolution.hippie.lan> <521BC472.7040804@freebsd.org> <521BD531.4090104@sbcglobal.net> <FF0E227A-0E15-4AFB-9BA0-E0E903D953F9@freebsd.org> <521C3EE4.80801@bitfrost.no> <3F762A16-3760-4FAA-B547-27529032AFEA@freebsd.org> <521C4CE3.9080400@freebsd.org> <627451FE-4F20-4E4D-9B24-59E0F340EF75@freebsd.org>
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On Aug 27, 2013, at 12:58 AM, Michael Tuexen wrote: > On Aug 27, 2013, at 8:53 AM, Andre Oppermann <andre@FreeBSD.org> = wrote: >=20 >> On 27.08.2013 08:30, Michael Tuexen wrote: >>> On Aug 27, 2013, at 7:53 AM, Hans Petter Selasky <hps@bitfrost.no> = wrote: >>>=20 >>>> On 08/27/13 00:38, Michael Tuexen wrote: >>>>> I did some tests with a small program. Having in struct pkthdr 64 = bit entities >>>>> results in a 64 bit alignment when used in struct mbuf. Using = __packed >>>>> for struct mbuf, removes the padding. >>>>=20 >>>>=20 >>>> Hi, >>>>=20 >>>> Maybe you could use __aligned(8) instead, and account for the extra = padding on all platforms? Packed has other disadvantages on ARM = platforms when accessing the structures, like that non-aligned access is = possible, and that it is sometimes slower than aligned access. >>> Isn't there a performance penalty when accessing 64-bit entities not = being 64-bit >>> aligned? If that is the case, wouldn't it make sense to add a 4 byte = padding to >>> struct m_hdr for ILP32? Then the problem should go away... >>=20 >> Either that or define MLEN and MHLEN in a way that actually reflects = the true >> size of what they are representing. The latter is the true bug. > Correct. There is the hidden assumption that there is no padding. = Maybe you can > put that in a comment... Maybe we can have it as a CTASSERT. Better than any comment. CTASSERTS = are free and catch this kind of thing... > We could also have some code (maybe under INVARIANTS) where we check = that > an struct mbuf has the size MSIZE and panic if not. > This would make things clearer if the problem happens again. >>=20 >>> We could also get rid of the 64 bit alignment by not having 64-bit = entities in >>> struct pkthdr. Removing sixtyfour should be easy. However, we now = have also >>> uint64_t csum_flags. >>=20 >> Yes, the 64 bit fields are to be used to store packet associated = information >> on its way through the stack. Reducing it to 32 bits would somewhat = defeat >> their purpose. > I completely agree... >>=20 >> --=20 >> Andre >>=20 >>=20 >=20 > _______________________________________________ > freebsd-arm@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-arm > To unsubscribe, send any mail to "freebsd-arm-unsubscribe@freebsd.org"
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