Re: [PATCH 4/21] i386 Broken bios common

[Zachary Amsden <zach@vmware.com>]

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 );