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Ingo Molnar

2007-Apr-18 17:49 UTC

[PATCH 4/21] i386 Broken bios common

* Zachary Amsden <zach@vmware.com> wrote:
>  	gdt = get_cpu_gdt_table(cpu);
>  	save_desc_40 = gdt[0x40 / 8];
> -	gdt[0x40 / 8] = bad_bios_desc;
> +	gdt[0x40 / 8] = gdt[GDT_ENTRY_BAD_BIOS_CACHE];
>  
i like the cleanup, but wouldnt it be simpler to dedicate GDT entry #8 
to the 0x40 descriptor, and hence be compatible with such broken BIOSes 
by default? Right now entry #8 is taken up by TLS segment #2, but we 
could change GDT_ENTRY_TLS_MIN from 6 to 9 and push the TLS segments to 
entries 9,10,11. [ Could there be any buggy SMM code that relies on 
having something at 0x40? ]

	Ingo

Linus Torvalds

2007-Apr-18 17:49 UTC

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[PATCH 4/21] i386 Broken bios common

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 ;(

		Linus

Zachary Amsden

2007-Apr-18 17:49 UTC

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[PATCH 4/21] i386 Broken bios common

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.

Signed-off-by: Zachary Amsden <zach@vmware.com>
Index: linux-2.6.14-zach-work/include/asm-i386/segment.h
==================================================================---
linux-2.6.14-zach-work.orig/include/asm-i386/segment.h	2005-11-04
15:46:10.000000000 -0800
+++ linux-2.6.14-zach-work/include/asm-i386/segment.h	2005-11-05
00:28:05.000000000 -0800
@@ -91,6 +91,15 @@
 #define GDT_ENTRY_BOOT_DS		(GDT_ENTRY_BOOT_CS + 1)
 #define __BOOT_DS	(GDT_ENTRY_BOOT_DS * 8)
 
+/*
+ * Turns out the BIOS data area at 0x400 is commonly accessed from broken
+ * BIOS using real mode selector 0x40.  We cached the bogus BIOS segment
+ * descriptor in a spare entry and fix it up at boot.
+ */
+#define GDT_ENTRY_BAD_BIOS_CACHE       4
+#define GDT_ENTRY_BAD_BIOS             (0x40 >> 3)
+#define BAD_BIOS_AREA                  (0x400)
+
 /* The PnP BIOS entries in the GDT */
 #define GDT_ENTRY_PNPBIOS_CS32		(GDT_ENTRY_PNPBIOS_BASE + 0)
 #define GDT_ENTRY_PNPBIOS_CS16		(GDT_ENTRY_PNPBIOS_BASE + 1)
Index: linux-2.6.14-zach-work/arch/i386/kernel/head.S
==================================================================---
linux-2.6.14-zach-work.orig/arch/i386/kernel/head.S	2005-11-04
15:46:50.000000000 -0800
+++ linux-2.6.14-zach-work/arch/i386/kernel/head.S	2005-11-05 00:28:12.000000000
-0800
@@ -487,7 +487,7 @@ ENTRY(cpu_gdt_table)
 	.quad 0x0000000000000000	/* 0x0b reserved */
 	.quad 0x0000000000000000	/* 0x13 reserved */
 	.quad 0x0000000000000000	/* 0x1b reserved */
-	.quad 0x0000000000000000	/* 0x20 unused */
+	.quad 0x0040920004000bff        /* 0x20 bad bios 3072 bytes at 0x400 */
 	.quad 0x0000000000000000	/* 0x28 unused */
 	.quad 0x0000000000000000	/* 0x33 TLS entry 1 */
 	.quad 0x0000000000000000	/* 0x3b TLS entry 2 */
Index: linux-2.6.14-zach-work/arch/i386/kernel/apm.c
==================================================================---
linux-2.6.14-zach-work.orig/arch/i386/kernel/apm.c	2005-11-04 15:46:50.000000000
-0800
+++ linux-2.6.14-zach-work/arch/i386/kernel/apm.c	2005-11-05 00:28:11.000000000
-0800
@@ -414,7 +414,6 @@ static DECLARE_WAIT_QUEUE_HEAD(apm_waitq
 static DECLARE_WAIT_QUEUE_HEAD(apm_suspend_waitqueue);
 static struct apm_user *	user_list;
 static DEFINE_SPINLOCK(user_list_lock);
-static struct desc_struct	bad_bios_desc = { 0, 0x00409200 };
 
 static char			driver_version[] = "1.16ac";	/* no spaces */
 
@@ -593,7 +592,7 @@ static u8 apm_bios_call(u32 func, u32 eb
 	cpu = get_cpu();
 	gdt = get_cpu_gdt_table(cpu);
 	save_desc_40 = gdt[0x40 / 8];
-	gdt[0x40 / 8] = bad_bios_desc;
+	gdt[0x40 / 8] = gdt[GDT_ENTRY_BAD_BIOS_CACHE];
 
 	local_save_flags(flags);
 	APM_DO_CLI;
@@ -637,7 +636,7 @@ static u8 apm_bios_call_simple(u32 func,
 	cpu = get_cpu();
 	gdt = get_cpu_gdt_table(cpu);
 	save_desc_40 = gdt[0x40 / 8];
-	gdt[0x40 / 8] = bad_bios_desc;
+	gdt[0x40 / 8] = gdt[GDT_ENTRY_BAD_BIOS_CACHE];
 
 	local_save_flags(flags);
 	APM_DO_CLI;
@@ -2275,15 +2274,6 @@ static int __init apm_init(void)
 	pm_active = 1;
 
 	/*
-	 * Set up a segment that references the real mode segment 0x40
-	 * that extends up to the end of page zero (that we have reserved).
-	 * This is for buggy BIOS's that refer to (real mode) segment 0x40
-	 * even though they are called in protected mode.
-	 */
-	set_base(bad_bios_desc, __va((unsigned long)0x40 << 4));
-	_set_limit((char *)&bad_bios_desc, 4095 - (0x40 << 4));
-
-	/*
 	 * Set up the long jump entry point to the APM BIOS, which is called
 	 * from inline assembly.
 	 */
Index: linux-2.6.14-zach-work/arch/i386/kernel/cpu/common.c
==================================================================---
linux-2.6.14-zach-work.orig/arch/i386/kernel/cpu/common.c	2005-11-04
15:46:10.000000000 -0800
+++ linux-2.6.14-zach-work/arch/i386/kernel/cpu/common.c	2005-11-05
00:28:09.000000000 -0800
@@ -596,6 +596,15 @@ void __devinit cpu_init(void)
 	 * and set up the GDT descriptor:
 	 */
  	memcpy(gdt, cpu_gdt_table, GDT_SIZE);
+ 
+	/*
+	 * Set up a segment that references the real mode segment 0x40
+	 * that extends up to the end of page zero (that we have reserved).
+	 * This is for buggy BIOS's that refer to (real mode) segment 0x40
+	 * even though they are called in protected mode.  The limit is
+	 * preset, we hardwire the base here.
+	 */
+	set_base(gdt[GDT_ENTRY_BAD_BIOS_CACHE], __va(BAD_BIOS_AREA));
 
 	/* Set up GDT entry for 16bit stack */
  	*(__u64 *)(&gdt[GDT_ENTRY_ESPFIX_SS]) |Index:
linux-2.6.14-zach-work/drivers/pnp/pnpbios/bioscalls.c
==================================================================---
linux-2.6.14-zach-work.orig/drivers/pnp/pnpbios/bioscalls.c	2005-11-04
15:46:10.000000000 -0800
+++ linux-2.6.14-zach-work/drivers/pnp/pnpbios/bioscalls.c	2005-11-05
00:28:12.000000000 -0800
@@ -70,8 +70,6 @@ set_base(per_cpu(cpu_gdt_table,cpu)[(sel
 set_limit(per_cpu(cpu_gdt_table,cpu)[(selname) >> 3], size); \
 } while(0)
 
-static struct desc_struct bad_bios_desc = { 0, 0x00409200 };
-
 /*
  * At some point we want to use this stack frame pointer to unwind
  * after PnP BIOS oopses.
@@ -107,7 +105,8 @@ static inline u16 call_pnp_bios(u16 func
 
 	cpu = get_cpu();
 	save_desc_40 = per_cpu(cpu_gdt_table,cpu)[0x40 / 8];
-	per_cpu(cpu_gdt_table,cpu)[0x40 / 8] = bad_bios_desc;
+	per_cpu(cpu_gdt_table,cpu)[0x40 / 8] = 
+		per_cpu(cpu_gdt_table,cpu)[GDT_ENTRY_BAD_BIOS_CACHE];
 
 	/* On some boxes IRQ's during PnP BIOS calls are deadly.  */
 	spin_lock_irqsave(&pnp_bios_lock, flags);
@@ -524,8 +523,6 @@ void pnpbios_calls_init(union pnp_bios_i
 	pnp_bios_callpoint.offset = header->fields.pm16offset;
 	pnp_bios_callpoint.segment = PNP_CS16;
 
-	set_base(bad_bios_desc, __va((unsigned long)0x40 << 4));
-	_set_limit((char *)&bad_bios_desc, 4095 - (0x40 << 4));
 	for(i=0; i < NR_CPUS; i++)
 	{
 		Q2_SET_SEL(i, PNP_CS32, &pnp_bios_callfunc, 64 * 1024);

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