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From: John Baldwin <jhb@freebsd.org>
To: Luigi Rizzo <luigi@freebsd.org>
Date: Mon, 30 Jul 2012 14:45:45 -0400
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Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org,
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Subject: Re: svn commit: r238765 - head/sys/dev/e1000
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On Wednesday, July 25, 2012 7:28:15 am Luigi Rizzo wrote:
> Author: luigi
> Date: Wed Jul 25 11:28:15 2012
> New Revision: 238765
> URL: http://svn.freebsd.org/changeset/base/238765
> 
> Log:
>   Use legacy interrupts as a default. This gives up to 10% speedup
>   when used in qemu (and this driver is for non-PCIe cards,
>   so probably its largest use is in virtualized environments).
>   
>   Approved by:	Jack Vogel
>   MFC after:	3 days

Why not make this a tunable or some such?  You could even have it only use the 
legacy handler under qemu easily enough.  There's no reason this has to be a 
compile-time option.  This is almost certainly slower on real hardware where 
this is important to work around dubious Intel Host-PCI bridges that result in 
aliased USB interrupts for every em(4) interrupt.

-- 
John Baldwin