| Message ID | 20190722213958.5761-5-julien.grall@arm.com |
|---|---|
| State | New |
| Headers | show |
| Series | xen/arm: Rework head.S to make it more compliant with the Arm Arm | expand |
On Mon, 22 Jul 2019, Julien Grall wrote: > Anything executed after the label common_start can be executed on all > CPUs. However most of the instructions executed between the label > common_start and init_uart are not executed on the boot CPU. > > The only instructions executed are to lookup the CPUID so it can be > printed on the console (if earlyprintk is enabled). Printing the CPUID > is not entirely useful to have for the boot CPU and requires a > conditional branch to bypass unused instructions. > > Furthermore, the function init_uart is only called for boot CPU > requiring another conditional branch. This makes the code a bit tricky > to follow. > > The UART initialization is now moved before the label common_start. This > now requires to have a slightly altered print for the boot CPU and set > the early UART base address in each the two path (boot CPU and > secondary CPUs). > > This has the nice effect to remove a couple of conditional branch in > the code. > > After this rework, the CPUID is only used at the very beginning of the > secondary CPUs boot path. So there is no need to "reserve" x24 for the > CPUID. > > Signed-off-by: Julien Grall <julien.grall@arm.com> Reviewed-by: Stefano Stabellini <sstabellini@kernel.org> and committed > --- > Changes in v2: > - Fold "xen/arm64: head: Don't "reserve" x24 for the CPUID" in > this patch > --- > xen/arch/arm/arm64/head.S | 31 ++++++++++++++++++++----------- > 1 file changed, 20 insertions(+), 11 deletions(-) > > diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S > index 6afe83c347..b684091aac 100644 > --- a/xen/arch/arm/arm64/head.S > +++ b/xen/arch/arm/arm64/head.S > @@ -69,7 +69,7 @@ > * x21 - DTB address (boot cpu only) > * x22 - is_secondary_cpu > * x23 - UART address > - * x24 - cpuid > + * x24 - > * x25 - identity map in place > * x26 - skip_zero_bss > * x27 - > @@ -265,6 +265,12 @@ real_start_efi: > load_paddr x21, _sdtb > #endif > > + /* Initialize the UART if earlyprintk has been enabled. */ > +#ifdef CONFIG_EARLY_PRINTK > + bl init_uart > +#endif > + PRINT("- Boot CPU booting -\r\n") > + > mov x22, #0 /* x22 := is_secondary_cpu */ > > b common_start > @@ -281,14 +287,11 @@ GLOBAL(init_secondary) > /* Boot CPU already zero BSS so skip it on secondary CPUs. */ > mov x26, #1 /* X26 := skip_zero_bss */ > > -common_start: > mrs x0, mpidr_el1 > ldr x13, =(~MPIDR_HWID_MASK) > bic x24, x0, x13 /* Mask out flags to get CPU ID */ > > - /* Non-boot CPUs wait here until __cpu_up is ready for them */ > - cbz x22, 1f > - > + /* Wait here until __cpu_up is ready to handle the CPU */ > load_paddr x0, smp_up_cpu > dsb sy > 2: ldr x1, [x0] > @@ -300,14 +303,14 @@ common_start: > > #ifdef CONFIG_EARLY_PRINTK > ldr x23, =EARLY_UART_BASE_ADDRESS /* x23 := UART base address */ > - cbnz x22, 1f > - bl init_uart /* Boot CPU sets up the UART too */ > -1: PRINT("- CPU ") > + PRINT("- CPU ") > mov x0, x24 > bl putn > PRINT(" booting -\r\n") > #endif > > +common_start: > + > PRINT("- Current EL ") > mrs x4, CurrentEL > mov x0, x4 > @@ -620,10 +623,16 @@ ENTRY(switch_ttbr) > ret > > #ifdef CONFIG_EARLY_PRINTK > -/* Bring up the UART. > - * x23: Early UART base address > - * Clobbers x0-x1 */ > +/* > + * Initialize the UART. Should only be called on the boot CPU. > + * > + * Ouput: > + * x23: Early UART base physical address > + * > + * Clobbers x0 - x1 > + */ > init_uart: > + ldr x23, =EARLY_UART_BASE_ADDRESS > #ifdef EARLY_PRINTK_INIT_UART > early_uart_init x23, 0 > #endif > -- > 2.11.0 >
diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S index 6afe83c347..b684091aac 100644 --- a/xen/arch/arm/arm64/head.S +++ b/xen/arch/arm/arm64/head.S @@ -69,7 +69,7 @@ * x21 - DTB address (boot cpu only) * x22 - is_secondary_cpu * x23 - UART address - * x24 - cpuid + * x24 - * x25 - identity map in place * x26 - skip_zero_bss * x27 - @@ -265,6 +265,12 @@ real_start_efi: load_paddr x21, _sdtb #endif + /* Initialize the UART if earlyprintk has been enabled. */ +#ifdef CONFIG_EARLY_PRINTK + bl init_uart +#endif + PRINT("- Boot CPU booting -\r\n") + mov x22, #0 /* x22 := is_secondary_cpu */ b common_start @@ -281,14 +287,11 @@ GLOBAL(init_secondary) /* Boot CPU already zero BSS so skip it on secondary CPUs. */ mov x26, #1 /* X26 := skip_zero_bss */ -common_start: mrs x0, mpidr_el1 ldr x13, =(~MPIDR_HWID_MASK) bic x24, x0, x13 /* Mask out flags to get CPU ID */ - /* Non-boot CPUs wait here until __cpu_up is ready for them */ - cbz x22, 1f - + /* Wait here until __cpu_up is ready to handle the CPU */ load_paddr x0, smp_up_cpu dsb sy 2: ldr x1, [x0] @@ -300,14 +303,14 @@ common_start: #ifdef CONFIG_EARLY_PRINTK ldr x23, =EARLY_UART_BASE_ADDRESS /* x23 := UART base address */ - cbnz x22, 1f - bl init_uart /* Boot CPU sets up the UART too */ -1: PRINT("- CPU ") + PRINT("- CPU ") mov x0, x24 bl putn PRINT(" booting -\r\n") #endif +common_start: + PRINT("- Current EL ") mrs x4, CurrentEL mov x0, x4 @@ -620,10 +623,16 @@ ENTRY(switch_ttbr) ret #ifdef CONFIG_EARLY_PRINTK -/* Bring up the UART. - * x23: Early UART base address - * Clobbers x0-x1 */ +/* + * Initialize the UART. Should only be called on the boot CPU. + * + * Ouput: + * x23: Early UART base physical address + * + * Clobbers x0 - x1 + */ init_uart: + ldr x23, =EARLY_UART_BASE_ADDRESS #ifdef EARLY_PRINTK_INIT_UART early_uart_init x23, 0 #endif
Anything executed after the label common_start can be executed on all CPUs. However most of the instructions executed between the label common_start and init_uart are not executed on the boot CPU. The only instructions executed are to lookup the CPUID so it can be printed on the console (if earlyprintk is enabled). Printing the CPUID is not entirely useful to have for the boot CPU and requires a conditional branch to bypass unused instructions. Furthermore, the function init_uart is only called for boot CPU requiring another conditional branch. This makes the code a bit tricky to follow. The UART initialization is now moved before the label common_start. This now requires to have a slightly altered print for the boot CPU and set the early UART base address in each the two path (boot CPU and secondary CPUs). This has the nice effect to remove a couple of conditional branch in the code. After this rework, the CPUID is only used at the very beginning of the secondary CPUs boot path. So there is no need to "reserve" x24 for the CPUID. Signed-off-by: Julien Grall <julien.grall@arm.com> --- Changes in v2: - Fold "xen/arm64: head: Don't "reserve" x24 for the CPUID" in this patch --- xen/arch/arm/arm64/head.S | 31 ++++++++++++++++++++----------- 1 file changed, 20 insertions(+), 11 deletions(-)