linux-kernel - Re: [PATCH] x86_64: fix problems due to use of "outb" to port 80 on some AMD64x2 laptops, etc.

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Message-ID: <4762AA68.9030507@qumranet.com>
Date:	Fri, 14 Dec 2007 18:08:08 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	"David P. Reed" <dpreed@...d.com>
CC:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Rene Herman <rene.herman@...access.nl>,
	Pavel Machek <pavel@....cz>,
	kvm-devel <kvm-devel@...ts.sourceforge.net>
Subject: Re: [PATCH] x86_64: fix problems due to use of "outb" to port 80
 on some AMD64x2 laptops, etc.

David P. Reed wrote:
> Replace use of outb to "unused" diagnostic port 0x80 for time delay
> with udelay based time delay on x86_64 architecture machines.  Fix for
> bugs 9511 and 6307 in bugzilla, plus bugs reported in
> bugzilla.redhat.com.
>
> Derived from suggestion (that didn't compile) by Pavel Machek, and
> tested, also based on measurements of typical timings of out's
> collated by Rene Herman from many in the community.
>
> This patch fixes a number of bugs known to cause problems on HP
> Pavilion dv9000z and dv6000z laptops - in the form of solid freezes
> when hwclock is used to show or set the time.  Also, it potentially
> improves bus utilization on SMP machines, by using a waiting process
> that doesn't tie up the ISA/LPC bus for 1 or 2 microseconds.
>

kvm will forward a virtual machine's writes to port 0x80 to the real 
port.  The reason is that the write is much faster than exiting and 
emulating it; the difference is measurable when compiling kernels.

Now if the cause is simply writing to port 0x80, then we must stop doing 
that.  But if the reason is the back-to-back writes, when we can keep 
it, since the other writes will be trapped by kvm and emulated.  Do you 
which is the case?

-- 
Any sufficiently difficult bug is indistinguishable from a feature.

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