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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Date:	Wed, 19 Dec 2007 17:03:29 +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:
> Avi Kivity wrote:
>> 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?
>>
> As for kvm, I don't have enough info to know anything about that.  Is 
> there a test you'd like me to try?
>

I have a test, but I see that it is broken for mainline.  I'll update it 
eventually, but...

> I think you are also asking if the crash on these laptops is caused 
> only by back-to-back writes.  Actually, it doesn't seem to matter if 
> they are back to back.  I can cause the crash if the writes to 80 are 
> very much spread out in time - it seems only to matter how many of 
> them get executed - almost as if there is a buffer overflow.  (And of 
> course if you do back to back writes to other ports that are 
> apparently fully unused, such as 0xED on my machine, no crash occurs).
>
> I believe (though no one seems to have confirming documentation from 
> the chipset or motherboard vendor) that port 80 is actually functional 
> for some unknown function on these machines.   (They do respond to 
> "in" instructions faster than a bus cycle abort does - more evidence).

That seems to be sufficient evidence for me to remove port 0x80 
pass-through from kvm and emulate it instead.  Given that port 80 writes 
take 1 microsecond, and that an in-kernel exit handler takes a similar 
amount of time, there won't be any significant performance loss.

-- 
error compiling committee.c: too many arguments to function

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