Date: Wed, 24 Oct 2001 20:34:19 -0500 (CDT) From: Jonathan Lemon <jlemon@flugsvamp.com> To: rizzo@aciri.org, net@freebsd.org Subject: Re: struct ifnet changes Message-ID: <200110250134.f9P1YJt18085@prism.flugsvamp.com> In-Reply-To: <local.mail.freebsd-stable/20011024171142.A48147@iguana.aciri.org>
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In article <local.mail.freebsd-stable/20011024171142.A48147@iguana.aciri.org> you write:
>Hi,
>in order to add polling support to network interfaces
>i need to add one more flag to network interface descriptors,
>but the relevant field in struct ifnet (if_flags) is only 16 bit
>wide and already fully used.
>
>I would like to extend it to 32 bit, which is not a problem in
>CURRENT, but doing this in STABLE will break binary compatibility
>with older drivers (presumably up to FreeBSD 4.2, as between 4.1
>and 4.2 there were other changes that probably prevent the use of
>old binary drivers).
>
>Are there strong objections to this change ?
>
>I can avoid breaking binary compatibility by using some other unused
>field (e.g. if_ipending, which is currently unused), but i'd rather
>not have to, because we risk to carry this dirty hack forever.
>
>If I am allowed to make changes to the structure, I would do the following:
>
> + change if_flags to an u_int32_t to accommodate more flags,
> and move it to the beginning of the structure -- this is
> accessed very very frequently, and this change makes the
> kernel 200 bytes smaller, and possibly a bit faster;
>
> + remove if_ipending -- noone is using it;
>
> + change if_index to u_int32_t (mostly to preserve alignment
> of the remaining fields);
>
> + maybe change if_unit and if_timer to 32 bit, for better alignment
> and code efficiency
>
> + redefine some of the (currently unused) fields for polling
> support. This does not compromise binary compatibility
> because the field size and position will remain the same,
> only the type will change.
>
>Comments ?
Hmm, I think you would be better off sending this to -net, not -stable. :-)
A few things:
- I've been kicking around the idea of splitting if_flags into two fields,
to better handle locking issues. Some of the fields are accessed by the
driver frequently (OACTIVE), while others are more static. However,
I haven't figured out whether this is needed or not.
- I also have a few changes for polling support of my own, I use the
poll_xmit/poll_recv slots in the dispatch table
- You mention moving if_flags to the first element, is there any code
that assumes that if_softc is the first element in the ifnet? Putting
at the start of the second cache line might be another option.
--
Jonathan
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