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.gr...@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,
--
Julien Grall
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