Linus Torvalds wrote:
>On Mon, 7 Nov 2005, Zachary Amsden wrote:
>
>
>>Both the APM BIOS and PnP BIOS code use a segment hack to simulate real
>>mode selector 0x40 (which points to the BIOS data area at 0x00400 in
>>real mode). Several broken BIOSen use selector 0x40 as if they were
>>running in real mode, which we make work by faking up selector 0x40 in
>>the GDT to point to physical memory starting at 0x400. We limit the
>>access to the remainder of this physical page using a byte granular
>>limit. Rather than have this tricky code in multiple places, it makes
>>sense to define it in one place, and the GDT makes a very convenient
>>place for it. Use GDT entry 4 as the BAD_BIOS_CACHE segment.
>>
>>
>
>I'd much rather use entry 8 instead, which should just automatically mean
>that selector 0x40 _always_ points to virtual address 0x400. No switching
>etc..
>
>Isn't this what Wine already has to work around, or something?
>
>Ingo, can we move the TLS selectors upwards, or does user space perhaps
>know about the current TLS layout? Wine in particular may well know ;(
>
>
Looking at the Wine code, it doesn't seem to know. It uses a GDT entry
if possible in preference to an LDT entry, and appears to not care which
TLS descriptor it gets. So I think it would be safe to reserve selector
0x40 for the BIOS real mode segment and move TLS up. That's kind of a
scary change, but I don't see anything wrong with it other than the
testing implications. Hopefully DOSEmu is not affected..
/***********************************************************************
* wine_ldt_alloc_fs
*
* Allocate an LDT entry for a %fs selector, reusing a global
* GDT selector if possible. Return the selector value.
*/
unsigned short wine_ldt_alloc_fs(void)
{
if (global_fs_sel == -1)
{
struct modify_ldt_s ldt_info;
int ret;
ldt_info.entry_number = -1; <--------- note it doesn't care
fill_modify_ldt_struct( &ldt_info, &null_entry );
if ((ret = set_thread_area( &ldt_info ) < 0))
{
global_fs_sel = 0; /* don't try it again */
if (errno != ENOSYS) perror( "set_thread_area" );
}
else global_fs_sel = (ldt_info.entry_number << 3) | 3;
}
if (global_fs_sel > 0) return global_fs_sel;
return wine_ldt_alloc_entries( 1 );
