[PATCH 14/21] i386 Apm is on cpu zero only

[PATCH 14/21] i386 Apm is on cpu zero only

[Zachary Amsden]

APM BIOS code has a protective wrapper that runs it only on CPU zero.  Thus,
no need to set APM BIOS segments in the GDT for other CPUs.

Signed-off-by: Zachary Amsden <zach@vmware.com>
Index: linux-2.6.14-zach-work/arch/i386/kernel/apm.c
===================================================================
--- linux-2.6.14-zach-work.orig/arch/i386/kernel/apm.c	2005-11-07 10:17:45.000000000 -0800
+++ linux-2.6.14-zach-work/arch/i386/kernel/apm.c	2005-11-07 13:36:05.000000000 -0800
@@ -2170,8 +2170,8 @@ static struct dmi_system_id __initdata a
 static int __init apm_init(void)
 {
 	struct proc_dir_entry *apm_proc;
+	struct desc_struct *gdt;
 	int ret;
-	int i;
 
 	dmi_check_system(apm_dmi_table);
 
@@ -2253,18 +2253,17 @@ static int __init apm_init(void)
 	 * not restrict themselves to their claimed limit.  When this happens,
 	 * they will cause a segmentation violation in the kernel at boot time.
 	 * Most BIOS's, however, will respect a 64k limit, so we use that.
+	 * 
+	 * Note we only set APM segments on CPU zero, since we pin the APM
+	 * code to that CPU.
 	 */
-	for (i = 0; i < NR_CPUS; i++) {
-		struct desc_struct *gdt = get_cpu_gdt_table(i);
- 		if (!gdt)
- 			continue;
-		set_base(&gdt[APM_CS >> 3],
-			 __va((unsigned long)apm_info.bios.cseg << 4));
-		set_base(&gdt[APM_CS_16 >> 3],
-			 __va((unsigned long)apm_info.bios.cseg_16 << 4));
-		set_base(&gdt[APM_DS >> 3],
-			 __va((unsigned long)apm_info.bios.dseg << 4));
-	}
+	gdt = get_cpu_gdt_table(0);
+	set_base(&gdt[APM_CS >> 3],
+		 __va((unsigned long)apm_info.bios.cseg << 4));
+	set_base(&gdt[APM_CS_16 >> 3],
+		 __va((unsigned long)apm_info.bios.cseg_16 << 4));
+	set_base(&gdt[APM_DS >> 3],
+		 __va((unsigned long)apm_info.bios.dseg << 4));
 
 	apm_proc = create_proc_info_entry("apm", 0, NULL, apm_get_info);
 	if (apm_proc)

Re: [PATCH 14/21] i386 Apm is on cpu zero only

[Ingo Molnar]

* Zachary Amsden <zach@vmware.com> wrote:

> APM BIOS code has a protective wrapper that runs it only on CPU zero.  
> Thus, no need to set APM BIOS segments in the GDT for other CPUs.

hm, do we want (need) to have that CPU#0 assumption forever?

	Ingo

Re: [PATCH 14/21] i386 Apm is on cpu zero only

[Zachary Amsden]

Ingo Molnar wrote:

>* Zachary Amsden <zach@vmware.com> wrote:
>
>  
>
>>APM BIOS code has a protective wrapper that runs it only on CPU zero.  
>>Thus, no need to set APM BIOS segments in the GDT for other CPUs.
>>    
>>
>
>hm, do we want (need) to have that CPU#0 assumption forever?
>  
>

Can't hurt, and APM is largely obsolete because of ACPI, so I'm only 
concerned with trimming and keeping adequate protection of the kernel 
from APM code while maintaining correctness.  I don't have a nice set of 
old machines with enough wacky APM BIOSen to validate that unpinning the 
CPU is ok.

Zach

Re: [PATCH 14/21] i386 Apm is on cpu zero only

[Alan Cox]

On Maw, 2005-11-08 at 04:53 -0800, Zachary Amsden wrote:
> Can't hurt, and APM is largely obsolete because of ACPI, so I'm only 
> concerned with trimming and keeping adequate protection of the kernel 
> from APM code while maintaining correctness.  I don't have a nice set of 
> old machines with enough wacky APM BIOSen to validate that unpinning the 
> CPU is ok.

A large number of SMP machines, probably the majority of APM based ones
require that APM calls occur on CPU#0. As I understand it from a BIOS
engineer involved in debugging that problem Redmond always does APM from
CPU #0 and may even guarantee it.

Alan

Re: [PATCH 14/21] i386 Apm is on cpu zero only

[Ingo Molnar]

* Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:

> On Maw, 2005-11-08 at 04:53 -0800, Zachary Amsden wrote:
> > Can't hurt, and APM is largely obsolete because of ACPI, so I'm only 
> > concerned with trimming and keeping adequate protection of the kernel 
> > from APM code while maintaining correctness.  I don't have a nice set of 
> > old machines with enough wacky APM BIOSen to validate that unpinning the 
> > CPU is ok.
> 
> A large number of SMP machines, probably the majority of APM based 
> ones require that APM calls occur on CPU#0. As I understand it from a 
> BIOS engineer involved in debugging that problem Redmond always does 
> APM from CPU #0 and may even guarantee it.

ok, then i'm all for making that more explicit - i.e. Zachary's patch is 
the right one.

	Ingo

Re: [PATCH 14/21] i386 Apm is on cpu zero only

[H. Peter Anvin]

Ingo Molnar wrote:
> * Zachary Amsden <zach@vmware.com> wrote:
> 
> 
>>APM BIOS code has a protective wrapper that runs it only on CPU zero.  
>>Thus, no need to set APM BIOS segments in the GDT for other CPUs.
> 
> hm, do we want (need) to have that CPU#0 assumption forever?
> 

APM BIOS should only ever run on the BSP, and I believe in Linux the BSP 
is always 0.  Since APM is obsolete, there is no future to consider.

	-hpa