Instead of clearing the bss area in assembly code, use the clear_bss()
function.

This requires to pass the start_info address as parameter to
xen_start_kernel() in order to avoid the xen_start_info being zeroed
again.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
 arch/x86/include/asm/setup.h |  3 +++
 arch/x86/kernel/head64.c     |  2 +-
 arch/x86/xen/enlighten_pv.c  |  8 ++++++--
 arch/x86/xen/xen-head.S      | 10 +---------
 4 files changed, 11 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index f8b9ee97a891..f37cbff7354c 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -120,6 +120,9 @@ void *extend_brk(size_t size, size_t align);
 	static char __brk_##name[size]
 
 extern void probe_roms(void);
+
+void clear_bss(void);
+
 #ifdef __i386__
 
 asmlinkage void __init i386_start_kernel(void);
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index bd4a34100ed0..e7e233209a8c 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -426,7 +426,7 @@ void __init do_early_exception(struct pt_regs *regs, int trapnr)
 
 /* Don't add a printk in there. printk relies on the PDA which is not initialized 
    yet. */
-static void __init clear_bss(void)
+void __init clear_bss(void)
 {
 	memset(__bss_start, 0,
 	       (unsigned long) __bss_stop - (unsigned long) __bss_start);
diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index e3297b15701c..70fb2ea85e90 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -1183,15 +1183,19 @@ static void __init xen_domu_set_legacy_features(void)
 extern void early_xen_iret_patch(void);
 
 /* First C function to be called on Xen boot */
-asmlinkage __visible void __init xen_start_kernel(void)
+asmlinkage __visible void __init xen_start_kernel(struct start_info *si)
 {
 	struct physdev_set_iopl set_iopl;
 	unsigned long initrd_start = 0;
 	int rc;
 
-	if (!xen_start_info)
+	if (!si)
 		return;
 
+	clear_bss();
+
+	xen_start_info = si;
+
 	__text_gen_insn(&early_xen_iret_patch,
 			JMP32_INSN_OPCODE, &early_xen_iret_patch, &xen_iret,
 			JMP32_INSN_SIZE);
diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 3a2cd93bf059..13af6fe453e3 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -48,15 +48,6 @@ SYM_CODE_START(startup_xen)
 	ANNOTATE_NOENDBR
 	cld
 
-	/* Clear .bss */
-	xor %eax,%eax
-	mov $__bss_start, %rdi
-	mov $__bss_stop, %rcx
-	sub %rdi, %rcx
-	shr $3, %rcx
-	rep stosq
-
-	mov %rsi, xen_start_info
 	mov initial_stack(%rip), %rsp
 
 	/* Set up %gs.
@@ -71,6 +62,7 @@ SYM_CODE_START(startup_xen)
 	cdq
 	wrmsr
 
+	mov	%rsi, %rdi
 	call xen_start_kernel
 SYM_CODE_END(startup_xen)
 	__FINIT
-- 
2.35.3

On 23.06.2022 11:46, Juergen Gross wrote:
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -1183,15 +1183,19 @@ static void __init xen_domu_set_legacy_features(void)
>  extern void early_xen_iret_patch(void);
>  
>  /* First C function to be called on Xen boot */
> -asmlinkage __visible void __init xen_start_kernel(void)
> +asmlinkage __visible void __init xen_start_kernel(struct start_info *si)
>  {
>  	struct physdev_set_iopl set_iopl;
>  	unsigned long initrd_start = 0;
>  	int rc;
>  
> -	if (!xen_start_info)
> +	if (!si)
>  		return;
>  
> +	clear_bss();

As per subsequent observation, this shouldn't really be needed: The
hypervisor (or tool stack for DomU-s) already does so. While I guess
we want to keep it to be on the safe side, maybe worth a comment?

Jan

On 23.06.22 11:51, Jan Beulich wrote:
> On 23.06.2022 11:46, Juergen Gross wrote:
>> --- a/arch/x86/xen/enlighten_pv.c
>> +++ b/arch/x86/xen/enlighten_pv.c
>> @@ -1183,15 +1183,19 @@ static void __init xen_domu_set_legacy_features(void)
>>   extern void early_xen_iret_patch(void);
>>   
>>   /* First C function to be called on Xen boot */
>> -asmlinkage __visible void __init xen_start_kernel(void)
>> +asmlinkage __visible void __init xen_start_kernel(struct start_info *si)
>>   {
>>   	struct physdev_set_iopl set_iopl;
>>   	unsigned long initrd_start = 0;
>>   	int rc;
>>   
>> -	if (!xen_start_info)
>> +	if (!si)
>>   		return;
>>   
>> +	clear_bss();
> 
> As per subsequent observation, this shouldn't really be needed: The
> hypervisor (or tool stack for DomU-s) already does so. While I guess
> we want to keep it to be on the safe side, maybe worth a comment?

Are you sure all possible boot loaders are clearing alloc-only sections?

I'd rather not count on e.g. grub doing this in all cases.


Juergen