Re: [Xen-devel] [PATCH v2 33/35] xen/arm32: head: Rework and document launch()

[Julien Grall <Julien.Grall@xxxxxxx>]

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@xxxxxxx>
>>
>> ---
>>      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,

-- 
Julien Grall
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