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

["linux-os (Dick Johnson)" <linux-os@analogic.com>]

On Fri, 14 Dec 2007, 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 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).
>
> I searched the DSDT to see if there is any evidence of an ACPI use for
> this port, but found nothing.
>
>

Attached is a patch that changes the outs to ins on port 0x80.
I did NOT let gcc decide what to do about modified registers.
Instead, the code saves/restores EAX itself so that all of the
times (whatever they are) are the same.

The code works and is running here. I also patched a very early
version (2.4.26) running on  a 400 MHz i486 with an real ISA
bus (Adaptec AHA1453). It works too.

David, will you please try it on your machine. Maybe reading
from the port is less harmful than writing.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.22.1 on an i686 machine (5588.27 BogoMips).
My book : http://www.AbominableFirebug.com/
_


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--- linux-2.6.22.1/include/asm-i386/io.h.orig	2007-07-10 14:56:30.000000000 -0400
+++ linux-2.6.22.1/include/asm-i386/io.h	2007-12-17 12:06:10.000000000 -0500
@@ -252,7 +252,10 @@
 
 static inline void native_io_delay(void)
 {
-	asm volatile("outb %%al,$0x80" : : : "memory");
+	asm volatile(	"pushl %%eax\n\t"
+			"inb $0x80, %%al\n\t"
+			"popl %%eax\n\t"
+			 : : : "memory");
 }
 
 #if defined(CONFIG_PARAVIRT)