Alexander van Heukelum wrote:
> On Thu, May 10, 2007 at 11:08:10AM -0700, H. Peter Anvin wrote:
>> As far as I could tell, "scan" simply caused the nonstandard video
>> driver scan modules (unsafe probes) to be invoked. Since those modules
>> are no longer present, there appeared to be no need for them. The VGA
>> and VESA probes are safe.
>
> It doesn't probe the hardware in dangerous ways. (Search for mode_scan
> in video.S) It works by trying to set a mode via the normal
> AH=0/AL=mode/int 0x10 method for all possible values of mode. It then
> checks if the bios reports the new mode as being set and reads a few
> standard vga registers to determine if it is a text mode. It's
> completely independent of the CONFIG_VIDEO_SVGA stuff.
It's dangerous, all right (which is why it doesn't do it by default),
since you have no guarantee that the BIOS doesn't totally vomit on these
calls -- or, like my laptop, take about a minute before giving up
finding nothing.
Anyway, I re-implemented scanning and pushed it out to the git tree;
please try it out as it does absolutely nothing on any of my machines.
> That makes me wonder: (from arch/i386/boot/pmjump.S)
>
> 37 movw $__BOOT_DS, %cx
> 38
> 39 movl %cr0, %edx
> 40 orb $1, %dl # Protected mode (PE) bit
> 41 movl %edx, %cr0
> 42
> 43 movw %cx, %ds
> 44 movw %cx, %es
> 45 movw %cx, %fs
> 46 movw %cx, %gs
> 47 movw %cx, %ss
> 48
> 49 # Jump to the 32-bit entrypoint
> 50 .byte 0x66, 0xea # ljmpl opcode
> 51 2: .long 0 # offset
> 52 .word __BOOT_CS # segment
>
> I thought the 32-bit jump was required to come before the segment loads.
> Does this code load values from the gdt, or are they just loaded as real
> mode segments? As long as it does not crash it does not matter, because
> head.S reloads them again.
Once CR0.PE is set, segments are loaded from the GDT.
-hpa
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
