Differences in emulation between kvm and kvm -no-kvm

Differences in emulation between kvm and kvm -no-kvm

[Milan Plzik]

  Hello,

  I somehow managed to produce code which behaves slightly differently
when using software emulation and when using kvm. As fas as I know, the
bug should be related to reading cursor position from VGA registers.

  Basically, the bug should be reproducible by executing:

	#define VGA_CURSOR_LOC_HIGH 0xe
	#define VGA_CURSOR_LOC_LOW  0xf

	priv.addr = 0x3d4;
	priv.data = 0x3d5;

	outb (priv.addr, VGA_CURSOR_LOC_HIGH);	/* Cursor location high */
	priv.cursor = inb (priv.data) << 8;

	outb (priv.addr, VGA_CURSOR_LOC_LOW); /* Cursor location low */
	priv.cursor += inb (priv.data);

  I put a testcase at
http://stashbox.org/manage_file/480477/kvm-bug.tar.gz . It's my school
project, so it's a bit more complicated; if neccessary, I can supply the
sources. In kvm -no-kvm it should cause reboot, in plain kvm it should
print few colored 'A's into the left upper corner of the screen (rest of
the code in binary is unreachable). It uses a bit more complicated setup
-- pxegrub2 and tftp loading, but that should not matter -- run.sh
should execute kvm with proper arguments, when executed from the kvm-bug
directory.

  Best regards,
	Milan

P.S:  Please Cc: me as I'm not subscribed to the list; when possible,
I'll be also idling at #kvm (nickname 'mmp').

Re: Differences in emulation between kvm and kvm -no-kvm

[Milan Plzik]

On Tuesday, 7. april 2009 at 21:59 +0200, Milan Plzik wrote:
> Hello,
> 
>   I somehow managed to produce code which behaves slightly differently
> when using software emulation and when using kvm. As fas as I know, the
> bug should be related to reading cursor position from VGA registers.
> 
>   Basically, the bug should be reproducible by executing:
> 
> 	#define VGA_CURSOR_LOC_HIGH 0xe
> 	#define VGA_CURSOR_LOC_LOW  0xf
> 
> 	priv.addr = 0x3d4;
> 	priv.data = 0x3d5;
> 
> 	outb (priv.addr, VGA_CURSOR_LOC_HIGH);	/* Cursor location high */
> 	priv.cursor = inb (priv.data) << 8;
> 
> 	outb (priv.addr, VGA_CURSOR_LOC_LOW); /* Cursor location low */
> 	priv.cursor += inb (priv.data);

  This is wrong; looks like the problem was in the end in improperly set
%esp register. Anyway, the problem is still the same -- kvm without
-no-kvm properly handled stack operations which shouldn't be
possible... . But sorry for sending incomplete info

> 
>   I put a testcase at
> http://stashbox.org/manage_file/480477/kvm-bug.tar.gz . It's my school
> project, so it's a bit more complicated; if neccessary, I can supply the
> sources. In kvm -no-kvm it should cause reboot, in plain kvm it should
> print few colored 'A's into the left upper corner of the screen (rest of
> the code in binary is unreachable). It uses a bit more complicated setup
> -- pxegrub2 and tftp loading, but that should not matter -- run.sh
> should execute kvm with proper arguments, when executed from the kvm-bug
> directory.
> 
>   Best regards,
> 	Milan
> 
> P.S:  Please Cc: me as I'm not subscribed to the list; when possible,
> I'll be also idling at #kvm (nickname 'mmp').