[LinuxBIOS] PATCH: relocate GDT before invoking payload

Steven J. Magnani steve at digidescorp.com
Fri Sep 9 18:34:56 CEST 2005

QNX Neutrino doesn't set up its own GDT right away. Its startup code has
some defensive logic that attempts to put the system into a known state.
This causes an exception when booted from LinuxBIOS, if its GDT got
trashed when elfboot() loaded the Neutrino image. See the hyperlink
below for more discussion.
The fix seems straightforward, and the increased amount of "table"
memory very modest. Do you object to this patch?

-----Original Message-----
From: yhlu [mailto:yinghailu at gmail.com] 
Sent: Friday, September 09, 2005 10:03 AM
To: Steven J. Magnani
Cc: linuxbios at openbios.org
Subject: Re: [LinuxBIOS] PATCH: relocate GDT before invoking payload


The payload will have its own GDT.


On 9/9/05, Steven J. Magnani <steve at digidescorp.com> wrote: 

I posted this patch previously; see
http://openbios.org/pipermail/linuxbios/2005-August/012063.html for the
original issue.

Here's another chance to comment before I commit it. 


--- src/arch/i386/boot/tables.c.orig    2005-09-08 13:35:39.125000000
+++ src/arch/i386/boot/tables.c 2005-08-16 16:17:41.766000000 -0500
@@ -7,6 +7,32 @@
#include <arch/acpi.h> 
#include "linuxbios_table.h"

+// Global Descriptor Table, defined in c_start.S
+extern uint8_t gdt;
+extern uint8_t gdt_end;
+/* i386 lgdt argument */
+struct gdtarg {
+    unsigned short limit; 
+    unsigned int base;
+} __attribute__((packed));
+// Copy GDT to new location and reload it
+// 2003-07 by SONE Takeshi
+// Ported from Etherboot to LinuxBIOS 2005-08 by Steve Magnani
+void move_gdt(unsigned long newgdt) 
+       uint16_t num_gdt_bytes = &gdt_end - &gdt;
+    struct gdtarg gdtarg;
+    printk_debug("Moving GDT to %#lx...", newgdt);
+    memcpy((void*)newgdt, &gdt, num_gdt_bytes); 
+    gdtarg.base = newgdt;
+    gdtarg.limit = num_gdt_bytes - 1;
+    __asm__ __volatile__ ("lgdt %0\n\t" : : "m" (gdtarg));
+    printk_debug("ok\n");
struct lb_memory *write_tables(void) 
        unsigned long low_table_start, low_table_end;
@@ -45,6 +71,10 @@
                low_table_end = 0x500;

+       // Relocate the GDT to reserved memory, so it won't get
+       move_gdt(low_table_end);
+       low_table_end += &gdt_end - &gdt;
        /* The linuxbios table must be in 0-4K or 960K-1M */
                              low_table_start, low_table_end,

Steven J. Magnani               "I claim this network for MARS!
www.digidescorp.com              Earthling, return my space modulator!"

#include <standard.disclaimer>

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