diff mbox series

[v2,33/35] xen/arm32: head: Rework and document launch()

Message ID 20190722213958.5761-34-julien.grall@arm.com (mailing list archive)
State Superseded
Headers show
Series xen/arm: Rework head.S to make it more compliant with the Arm Arm | expand

Commit Message

Julien Grall July 22, 2019, 9:39 p.m. UTC
Boot CPU and secondary CPUs will use different entry point to C code. At
the moment, the decision on which entry to use is taken within launch().

In order to avoid using conditional instruction and make the call
clearer, launch() is reworked to take in parameters the entry point and its
arguments.

Lastly, document the behavior and the main registers usage within the
function.

Signed-off-by: Julien Grall <julien.grall@arm.com>

---
    Changes in v2:
        - Patch added
---
 xen/arch/arm/arm32/head.S | 34 ++++++++++++++++++++++++----------
 1 file changed, 24 insertions(+), 10 deletions(-)

Comments

Stefano Stabellini July 30, 2019, 9:21 p.m. UTC | #1
On Mon, 22 Jul 2019, Julien Grall wrote:
> Boot CPU and secondary CPUs will use different entry point to C code. At
> the moment, the decision on which entry to use is taken within launch().
> 
> In order to avoid using conditional instruction and make the call
> clearer, launch() is reworked to take in parameters the entry point and its
> arguments.
> 
> Lastly, document the behavior and the main registers usage within the
> function.
> 
> Signed-off-by: Julien Grall <julien.grall@arm.com>
> 
> ---
>     Changes in v2:
>         - Patch added
> ---
>  xen/arch/arm/arm32/head.S | 34 ++++++++++++++++++++++++----------
>  1 file changed, 24 insertions(+), 10 deletions(-)
> 
> diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
> index e0f8c2e0cb..6d55a2119a 100644
> --- a/xen/arch/arm/arm32/head.S
> +++ b/xen/arch/arm/arm32/head.S
> @@ -170,6 +170,11 @@ primary_switched:
>          /* Use a virtual address to access the UART. */
>          mov_w r11, EARLY_UART_VIRTUAL_ADDRESS
>  #endif
> +        PRINT("- Ready -\r\n")
> +        /* Setup the arguments for start_xen and jump to C world */
> +        mov   r0, r10                /* r0 := Physical offset */
> +        mov   r1, r8                 /* r1 := paddr(FDT) */
> +        ldr   r2, =start_xen
>          b     launch
>  ENDPROC(start)
>  
> @@ -234,6 +239,9 @@ secondary_switched:
>          /* Use a virtual address to access the UART. */
>          mov_w r11, EARLY_UART_VIRTUAL_ADDRESS
>  #endif
> +        PRINT("- Ready -\r\n")
> +        /* Jump to C world */
> +        ldr   r2, =start_secondary
>          b     launch
>  ENDPROC(init_secondary)
>  
> @@ -578,19 +586,25 @@ setup_fixmap:
>          mov   pc, lr
>  ENDPROC(setup_fixmap)
>  
> +/*
> + * Setup the initial stack and jump to the C world
> + *
> + * Inputs:
> + *   r0 : Argument 0 of the C function to call
> + *   r1 : Argument 1 of the C function to call
> + *   r2 : C entry point
> + *
> + * Clobbers r3
> + */
>  launch:
> -        PRINT("- Ready -\r\n")
> -
> -        ldr   r0, =init_data
> -        add   r0, #INITINFO_stack    /* Find the boot-time stack */
> -        ldr   sp, [r0]
> +        ldr   r3, =init_data
> +        add   r3, #INITINFO_stack    /* Find the boot-time stack */
> +        ldr   sp, [r3]
>          add   sp, #STACK_SIZE        /* (which grows down from the top). */
>          sub   sp, #CPUINFO_sizeof    /* Make room for CPU save record */
> -        teq   r12, #0
> -        moveq r0, r10                /* Marshal args: - phys_offset */
> -        moveq r1, r8                 /*               - DTB address */
> -        beq   start_xen              /* and disappear into the land of C */
> -        b     start_secondary        /* (to the appropriate entry point) */
> +
> +        /* Jump to C world */
> +       bx    r2

Why bx?


>  ENDPROC(launch)
>  
>  /* Fail-stop */
> -- 
> 2.11.0
>
Julien Grall July 30, 2019, 9:34 p.m. UTC | #2
On 30/07/2019 22:21, Stefano Stabellini wrote:
> On Mon, 22 Jul 2019, Julien Grall wrote:
>> Boot CPU and secondary CPUs will use different entry point to C code. At
>> the moment, the decision on which entry to use is taken within launch().
>>
>> In order to avoid using conditional instruction and make the call
>> clearer, launch() is reworked to take in parameters the entry point and its
>> arguments.
>>
>> Lastly, document the behavior and the main registers usage within the
>> function.
>>
>> Signed-off-by: Julien Grall <julien.grall@arm.com>
>>
>> ---
>>      Changes in v2:
>>          - Patch added
>> ---
>>   xen/arch/arm/arm32/head.S | 34 ++++++++++++++++++++++++----------
>>   1 file changed, 24 insertions(+), 10 deletions(-)
>>
>> diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
>> index e0f8c2e0cb..6d55a2119a 100644
>> --- a/xen/arch/arm/arm32/head.S
>> +++ b/xen/arch/arm/arm32/head.S
>> @@ -170,6 +170,11 @@ primary_switched:
>>           /* Use a virtual address to access the UART. */
>>           mov_w r11, EARLY_UART_VIRTUAL_ADDRESS
>>   #endif
>> +        PRINT("- Ready -\r\n")
>> +        /* Setup the arguments for start_xen and jump to C world */
>> +        mov   r0, r10                /* r0 := Physical offset */
>> +        mov   r1, r8                 /* r1 := paddr(FDT) */
>> +        ldr   r2, =start_xen
>>           b     launch
>>   ENDPROC(start)
>>   
>> @@ -234,6 +239,9 @@ secondary_switched:
>>           /* Use a virtual address to access the UART. */
>>           mov_w r11, EARLY_UART_VIRTUAL_ADDRESS
>>   #endif
>> +        PRINT("- Ready -\r\n")
>> +        /* Jump to C world */
>> +        ldr   r2, =start_secondary
>>           b     launch
>>   ENDPROC(init_secondary)
>>   
>> @@ -578,19 +586,25 @@ setup_fixmap:
>>           mov   pc, lr
>>   ENDPROC(setup_fixmap)
>>   
>> +/*
>> + * Setup the initial stack and jump to the C world
>> + *
>> + * Inputs:
>> + *   r0 : Argument 0 of the C function to call
>> + *   r1 : Argument 1 of the C function to call
>> + *   r2 : C entry point
>> + *
>> + * Clobbers r3
>> + */
>>   launch:
>> -        PRINT("- Ready -\r\n")
>> -
>> -        ldr   r0, =init_data
>> -        add   r0, #INITINFO_stack    /* Find the boot-time stack */
>> -        ldr   sp, [r0]
>> +        ldr   r3, =init_data
>> +        add   r3, #INITINFO_stack    /* Find the boot-time stack */
>> +        ldr   sp, [r3]
>>           add   sp, #STACK_SIZE        /* (which grows down from the top). */
>>           sub   sp, #CPUINFO_sizeof    /* Make room for CPU save record */
>> -        teq   r12, #0
>> -        moveq r0, r10                /* Marshal args: - phys_offset */
>> -        moveq r1, r8                 /*               - DTB address */
>> -        beq   start_xen              /* and disappear into the land of C */
>> -        b     start_secondary        /* (to the appropriate entry point) */
>> +
>> +        /* Jump to C world */
>> +       bx    r2
> 
> Why bx?
The only two other possible instructions would be:
    1) blx r2: we don't need to save the return address
    2) mov pc, r2: The Arm Arm recommends to use bx/blx instead of this.

So bx seems the best fit. Any other suggestion?

Also, I would probably replace all the "mov pc, lr" I added with "bx lr".

Cheers,
Stefano Stabellini July 31, 2019, 8:27 p.m. UTC | #3
On Tue, 30 Jul 2019, Julien Grall wrote:
> On 30/07/2019 22:21, Stefano Stabellini wrote:
> > On Mon, 22 Jul 2019, Julien Grall wrote:
> >> Boot CPU and secondary CPUs will use different entry point to C code. At
> >> the moment, the decision on which entry to use is taken within launch().
> >>
> >> In order to avoid using conditional instruction and make the call
> >> clearer, launch() is reworked to take in parameters the entry point and its
> >> arguments.
> >>
> >> Lastly, document the behavior and the main registers usage within the
> >> function.
> >>
> >> Signed-off-by: Julien Grall <julien.grall@arm.com>
> >>
> >> ---
> >>      Changes in v2:
> >>          - Patch added
> >> ---
> >>   xen/arch/arm/arm32/head.S | 34 ++++++++++++++++++++++++----------
> >>   1 file changed, 24 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
> >> index e0f8c2e0cb..6d55a2119a 100644
> >> --- a/xen/arch/arm/arm32/head.S
> >> +++ b/xen/arch/arm/arm32/head.S
> >> @@ -170,6 +170,11 @@ primary_switched:
> >>           /* Use a virtual address to access the UART. */
> >>           mov_w r11, EARLY_UART_VIRTUAL_ADDRESS
> >>   #endif
> >> +        PRINT("- Ready -\r\n")
> >> +        /* Setup the arguments for start_xen and jump to C world */
> >> +        mov   r0, r10                /* r0 := Physical offset */
> >> +        mov   r1, r8                 /* r1 := paddr(FDT) */
> >> +        ldr   r2, =start_xen
> >>           b     launch
> >>   ENDPROC(start)
> >>   
> >> @@ -234,6 +239,9 @@ secondary_switched:
> >>           /* Use a virtual address to access the UART. */
> >>           mov_w r11, EARLY_UART_VIRTUAL_ADDRESS
> >>   #endif
> >> +        PRINT("- Ready -\r\n")
> >> +        /* Jump to C world */
> >> +        ldr   r2, =start_secondary
> >>           b     launch
> >>   ENDPROC(init_secondary)
> >>   
> >> @@ -578,19 +586,25 @@ setup_fixmap:
> >>           mov   pc, lr
> >>   ENDPROC(setup_fixmap)
> >>   
> >> +/*
> >> + * Setup the initial stack and jump to the C world
> >> + *
> >> + * Inputs:
> >> + *   r0 : Argument 0 of the C function to call
> >> + *   r1 : Argument 1 of the C function to call
> >> + *   r2 : C entry point
> >> + *
> >> + * Clobbers r3
> >> + */
> >>   launch:
> >> -        PRINT("- Ready -\r\n")
> >> -
> >> -        ldr   r0, =init_data
> >> -        add   r0, #INITINFO_stack    /* Find the boot-time stack */
> >> -        ldr   sp, [r0]
> >> +        ldr   r3, =init_data
> >> +        add   r3, #INITINFO_stack    /* Find the boot-time stack */
> >> +        ldr   sp, [r3]
> >>           add   sp, #STACK_SIZE        /* (which grows down from the top). */
> >>           sub   sp, #CPUINFO_sizeof    /* Make room for CPU save record */
> >> -        teq   r12, #0
> >> -        moveq r0, r10                /* Marshal args: - phys_offset */
> >> -        moveq r1, r8                 /*               - DTB address */
> >> -        beq   start_xen              /* and disappear into the land of C */
> >> -        b     start_secondary        /* (to the appropriate entry point) */
> >> +
> >> +        /* Jump to C world */
> >> +       bx    r2
> > 
> > Why bx?
> The only two other possible instructions would be:
>     1) blx r2: we don't need to save the return address
>     2) mov pc, r2: The Arm Arm recommends to use bx/blx instead of this.
> 
> So bx seems the best fit. Any other suggestion?
> 
> Also, I would probably replace all the "mov pc, lr" I added with "bx lr".

There is really no alternative to bx. I forgot that "b" doesn't support
a register as an operand.

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
diff mbox series

Patch

diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
index e0f8c2e0cb..6d55a2119a 100644
--- a/xen/arch/arm/arm32/head.S
+++ b/xen/arch/arm/arm32/head.S
@@ -170,6 +170,11 @@  primary_switched:
         /* Use a virtual address to access the UART. */
         mov_w r11, EARLY_UART_VIRTUAL_ADDRESS
 #endif
+        PRINT("- Ready -\r\n")
+        /* Setup the arguments for start_xen and jump to C world */
+        mov   r0, r10                /* r0 := Physical offset */
+        mov   r1, r8                 /* r1 := paddr(FDT) */
+        ldr   r2, =start_xen
         b     launch
 ENDPROC(start)
 
@@ -234,6 +239,9 @@  secondary_switched:
         /* Use a virtual address to access the UART. */
         mov_w r11, EARLY_UART_VIRTUAL_ADDRESS
 #endif
+        PRINT("- Ready -\r\n")
+        /* Jump to C world */
+        ldr   r2, =start_secondary
         b     launch
 ENDPROC(init_secondary)
 
@@ -578,19 +586,25 @@  setup_fixmap:
         mov   pc, lr
 ENDPROC(setup_fixmap)
 
+/*
+ * Setup the initial stack and jump to the C world
+ *
+ * Inputs:
+ *   r0 : Argument 0 of the C function to call
+ *   r1 : Argument 1 of the C function to call
+ *   r2 : C entry point
+ *
+ * Clobbers r3
+ */
 launch:
-        PRINT("- Ready -\r\n")
-
-        ldr   r0, =init_data
-        add   r0, #INITINFO_stack    /* Find the boot-time stack */
-        ldr   sp, [r0]
+        ldr   r3, =init_data
+        add   r3, #INITINFO_stack    /* Find the boot-time stack */
+        ldr   sp, [r3]
         add   sp, #STACK_SIZE        /* (which grows down from the top). */
         sub   sp, #CPUINFO_sizeof    /* Make room for CPU save record */
-        teq   r12, #0
-        moveq r0, r10                /* Marshal args: - phys_offset */
-        moveq r1, r8                 /*               - DTB address */
-        beq   start_xen              /* and disappear into the land of C */
-        b     start_secondary        /* (to the appropriate entry point) */
+
+        /* Jump to C world */
+       bx    r2
 ENDPROC(launch)
 
 /* Fail-stop */