[Qemu-devel] [PATCH] extboot: properly set int 0x13 return value

[Qemu-devel] [PATCH] extboot: properly set int 0x13 return value

[Glauber Costa]

Callers of int 0x13 usually rely on the carry flag being
clear/set to indicate the status of the interrupt execution.

However, our current code clear or set the flags register,
which is totally useless. Whichever value it has, will
be overwritten by the flags value _before_ the interrupt, due to
the iret instruction.

This fixes a bug that prevents slackware (and possibly win2k, untested)
to boot.

Signed-off-by: Glauber Costa <glommer@redhat.com>
---
 extboot/extboot.S |   52 ++++++++++++++++++++++++++--------------------------
 1 files changed, 26 insertions(+), 26 deletions(-)

diff --git a/extboot/extboot.S b/extboot/extboot.S
index 2630abb..4cbfe11 100644
--- a/extboot/extboot.S
+++ b/extboot/extboot.S
@@ -99,24 +99,24 @@ int19_handler:
 
 #define FLAGS_CF	0x01
 
-.macro clc
-	push %ax
-	pushf
-	pop %ax
-	and $(~FLAGS_CF), %ax
-	push %ax
-	popf
-	pop %ax
+/* The two macro below clear/set the carry flag to indicate the status
+ * of the interrupt execution. It is not enough to issue a clc/stc instruction, 
+ * since the value of the flags register will be overwritten by whatever is
+ * in the stack frame
+ */
+.macro clc_stack
+	push %bp
+	mov %sp, %bp
+	/* 8 = 2 (bp, just pushed) + 2 (ip) + 3 (real mode interrupt frame)
+	and $(~FLAGS_CF), 8(%bp)
+	pop %bp
 .endm
 
-.macro stc
-	push %ax
-	pushf
-	pop %ax
-	or $(FLAGS_CF), %ax
-	push %ax
-	popf
-	pop %ax
+.macro stc_stack
+	push %bp
+	/* 8 = 2 (bp, just pushed) + 2 (ip) + 3 (real mode interrupt frame)
+	or $(FLAGS_CF), 8(%bp)
+	pop %bp
 .endm
 
 /* we clobber %bx */
@@ -292,7 +292,7 @@ mul32:  /* lo,      hi,     lo,     hi */
 
 disk_reset:
 	movb $0, %ah
-	clc
+	clc_stack
 	ret
 
 /* this really should be a function, not a macro but i'm lazy */
@@ -395,7 +395,7 @@ disk_reset:
 	pop %ax
 
 	mov $0, %ah
-	clc
+	clc_stack
 	ret
 .endm
 
@@ -454,12 +454,12 @@ read_disk_drive_parameters:
 	pop %bx
 
 	/* do this last since it's the most sensitive */
-	clc
+	clc_stack
 	ret
 
 alternate_disk_reset:
 	movb $0, %ah
-	clc
+	clc_stack
 	ret
 
 read_disk_drive_size:
@@ -498,21 +498,21 @@ read_disk_drive_size:
 	freea
 	pop %bx
 
-	clc
+	clc_stack
 	ret
 
 check_if_extensions_present:
 	mov $0x30, %ah
 	mov $0xAA55, %bx
 	mov $0x07, %cx
-	clc
+	clc_stack
 	ret
 
 .macro extended_read_write_sectors cmd
 	cmpb $10, 0(%si)
 	jg 1f
 	mov $1, %ah
-	stc
+	stc_stack
 	ret
 1:
 	push %ax
@@ -544,7 +544,7 @@ check_if_extensions_present:
 	pop %ax
 
 	mov $0, %ah
-	clc
+	clc_stack
 	ret
 .endm
 
@@ -612,12 +612,12 @@ get_extended_drive_parameters:
 	pop %ax
 
 	mov $0, %ah
-	clc
+	clc_stack
 	ret
 
 terminate_disk_emulation:
 	mov $1, %ah
-	stc
+	stc_stack
 	ret
 
 int13_handler:
-- 
1.5.6.5

[Qemu-devel] Re: [PATCH] extboot: properly set int 0x13 return value

[Anthony Liguori]

Glauber Costa wrote:
> Callers of int 0x13 usually rely on the carry flag being
> clear/set to indicate the status of the interrupt execution.
>
> However, our current code clear or set the flags register,
> which is totally useless. Whichever value it has, will
> be overwritten by the flags value _before_ the interrupt, due to
> the iret instruction.
>
> This fixes a bug that prevents slackware (and possibly win2k, untested)
> to boot.
>   

Good catch!

> Signed-off-by: Glauber Costa <glommer@redhat.com>
>
>   

Acked-by: Anthony Liguori <aliguori@us.ibm.com>

Regards,

Anthony Liguori

[Qemu-devel] Re: [PATCH] extboot: properly set int 0x13 return value

[Avi Kivity]

Glauber Costa wrote:
> Callers of int 0x13 usually rely on the carry flag being
> clear/set to indicate the status of the interrupt execution.
>
> However, our current code clear or set the flags register,
> which is totally useless. Whichever value it has, will
> be overwritten by the flags value _before_ the interrupt, due to
> the iret instruction.
>
> This fixes a bug that prevents slackware (and possibly win2k, untested)
> to boot.
>
>   

Applied, thanks.

>  
> -.macro clc
> -	push %ax
> -	pushf
> -	pop %ax
> -	and $(~FLAGS_CF), %ax
> -	push %ax
> -	popf
> -	pop %ax
>   

Anthony, any reason you did not use the 'clc' instruction instead of a 
macro?

-- 
error compiling committee.c: too many arguments to function

[Qemu-devel] Re: [PATCH] extboot: properly set int 0x13 return value

[Anthony Liguori]

Avi Kivity wrote:
> Applied, thanks.
>
>>  
>> -.macro clc
>> -    push %ax
>> -    pushf
>> -    pop %ax
>> -    and $(~FLAGS_CF), %ax
>> -    push %ax
>> -    popf
>> -    pop %ax
>>   
>
> Anthony, any reason you did not use the 'clc' instruction instead of a 
> macro?

Propensity for pain?

I have no idea.  I assume I had a reason at the time.  Probably debug 
related.

Regards,

Anthony Liguori