Re: [PATCH] Secondary cpus boot-up for non defalut location builtkernels

[Vivek Goyal]

> > This patch fixes the problem of secondary cpus boot up. This situation
> > is faced when kernel is built for non default locations like 16MB and
> > onwards. In this configuration, only primary cpu (BP) comes and
> > secondary cpus don't boot.
> > 
> > Problem occurs because in trampoline code, lgdt is not able to load the
> > GDT as it happens to be situated beyond 16MB. This is due to the fact
> > that cpu is still in real mode and default operand size is 16bit.
> 
> Which means that the cpu will load a 24bit linear address (3 bytes)
> because it is acting like a 286.
>  
> > This patch uses lgdtl instead of lgdt to force operand size to 32
> > instead of 16.
> 
> Sounds sane looking at the trampoline I suspect that people thought
> it was using a 32bit address all along.  And the code only worked
> because the data was stored little endian.  

> We probably want to apply
> the same fix to the ldt instruction just about it.  Just in case
> we ever decide to populate an idt at that point.


Sounds good. Sending the patch again with suggested change included.

> 
> But the fix certainly looks good from here.
> 
> Eric
> 

-- 
Vivek Goyal
Linux Technology Center
India Software Labs
IBM India, Bangalore

This patch fixes the problem of secondary cpus not coming up over a reboot. 
This problem was seen when a kernel compiled for non default (16MB) location 
is booted.

Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
---

 linux-2.6.10-rc3-mm1-changes-root/arch/i386/kernel/trampoline.S |   10 ++++++++--
 1 files changed, 8 insertions(+), 2 deletions(-)

diff -puN arch/i386/kernel/trampoline.S~boot_ap_for_nondefault_kernel arch/i386/kernel/trampoline.S
--- linux-2.6.10-rc3-mm1-changes/arch/i386/kernel/trampoline.S~boot_ap_for_nondefault_kernel	2004-12-22 16:36:50.000000000 +0530
+++ linux-2.6.10-rc3-mm1-changes-root/arch/i386/kernel/trampoline.S	2004-12-22 21:51:35.000000000 +0530
@@ -51,8 +51,14 @@ r_base = .
 	movl	$0xA5A5A5A5, trampoline_data - r_base
 				# write marker for master knows we're running
 
-	lidt	boot_idt - r_base	# load idt with 0, 0
-	lgdt	boot_gdt - r_base	# load gdt with whatever is appropriate
+	/* GDT tables in non default location kernel can be beyond 16MB and
+	 * lgdt will not be able to load the address as in real mode default
+	 * operand size is 16bit. Use lgdtl instead to force operand size
+	 * to 32 bit.
+	 */
+
+	lidtl	boot_idt - r_base	# load idt with 0, 0
+	lgdtl	boot_gdt - r_base	# load gdt with whatever is appropriate
 
 	xor	%ax, %ax
 	inc	%ax		# protected mode (PE) bit
_