Re: [PATCH] x86_64: fix problems due to use of "outb" to port 80on some AMD64x2 laptops, etc.
From: Avi Kivity
Date: Wed Dec 19 2007 - 10:03:46 EST
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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