[Xen-devel] [PATCH v2 08/35] xen/arm64: head: Rework and document zero_bss()

[Julien Grall <julien.grall@xxxxxxx>]

On secondary CPUs, zero_bss() will be a NOP because BSS only need to be
zeroed once at boot. So the call in the secondary CPUs path can be
removed. It also means that x26 does not need to be set for secondary
CPU.

Note that we will need to keep x26 around for the boot CPU as BSS should
not be reset when booting via UEFI.

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

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

---
    Changes in v2:
        - Clarify the commit message
        - Mention x20 is used as an input
---
 xen/arch/arm/arm64/head.S | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index fbcc792ade..92c8338d71 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -71,7 +71,7 @@
  *  x23 - UART address
  *  x24 -
  *  x25 - identity map in place
- *  x26 - skip_zero_bss
+ *  x26 - skip_zero_bss (boot cpu only)
  *  x27 -
  *  x28 -
  *  x29 -
@@ -313,8 +313,6 @@ GLOBAL(init_secondary)
         sub   x20, x19, x0           /* x20 := phys-offset */
 
         mov   x22, #1                /* x22 := is_secondary_cpu */
-        /* Boot CPU already zero BSS so skip it on secondary CPUs. */
-        mov   x26, #1                /* X26 := skip_zero_bss */
 
         mrs   x0, mpidr_el1
         ldr   x13, =(~MPIDR_HWID_MASK)
@@ -337,7 +335,6 @@ GLOBAL(init_secondary)
         PRINT(" booting -\r\n")
 #endif
         bl    check_cpu_mode
-        bl    zero_bss
         bl    cpu_init
         bl    create_page_tables
         bl    enable_mmu
@@ -375,6 +372,15 @@ check_cpu_mode:
         b fail
 ENDPROC(check_cpu_mode)
 
+/*
+ * Zero BSS
+ *
+ * Inputs:
+ *   x20: Physical offset
+ *   x26: Do we need to zero BSS?
+ *
+ * Clobbers x0 - x3
+ */
 zero_bss:
         /* Zero BSS only when requested */
         cbnz  x26, skip_bss
-- 
2.11.0


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