Re: [PATCH 3/5] arm: decompressor: define a new zImage tag

It was <2020-06-01 pon 15:55>, when Russell King - ARM Linux admin wrote:

On Mon, Jun 01, 2020 at 04:27:52PM +0200, Łukasz Stelmach wrote:

quoted

Add DCSZ tag which holds dynamic memory (stack, bss, malloc pool)
requirements of the decompressor code.

Why do we need to know the stack and BSS size, when the userspace
kexec tool doesn't need to know this to perform the same function.

kexec-tools load zImage as low in DRAM as possible and rely on two
assumptions:

    + the zImage will copy itself to make enough room for the kernel,
    + sizeof(zImage+mem) < sizeof(kernel+mem), which is true because
      of compression.

       DRAM start
       + 0x8000

zImage    |-----------|-----|-------|
            text+data   bss   stack 

                 text+data           bss   
kernel    |---------------------|-------------------|


initrd                                              |-initrd-|-dtb-|


My code on the other hand tries to load the zImage high enough to make
room for the kernel without copying the zImage.


            text+data           bss   
kernel    |-----------|-------------------|

zImage                |-----------|-----|-------|
                        text+data   bss   stack 

initrd                                          |-initrd-|-dtb-|


In such case the second assumption would be

    sizeof(zImage+mem) < sizeof(kernel bss)

and I can't tell for sure it would always be true. That is why we need
detailed information about decompressor memory requirements.

quoted

Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
---
 arch/arm/boot/compressed/vmlinux.lds.S |  9 ++++++++-
 arch/arm/include/asm/image.h           | 13 +++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/arm/boot/compressed/vmlinux.lds.S b/arch/arm/boot/compressed/vmlinux.lds.S
index 308e9cd6a897..dcfdb3209c90 100644
--- a/arch/arm/boot/compressed/vmlinux.lds.S
+++ b/arch/arm/boot/compressed/vmlinux.lds.S

@@ -39,6 +39,11 @@ SECTIONS
     LONG(ARM_ZIMAGE_MAGIC3)
     LONG(ZIMAGE_MAGIC(__piggy_size_addr - _start))
     LONG(ZIMAGE_MAGIC(_kernel_bss_size))
+    LONG(ZIMAGE_MAGIC(5))
+    LONG(ARM_ZIMAGE_MAGIC4)
+    LONG(ZIMAGE_MAGIC(_end - __bss_start))
+    LONG(ZIMAGE_MAGIC(_stack_end - _stack_start))
+    LONG(ZIMAGE_MAGIC(_malloc_size))
     LONG(0)
     _table_end = .;
   }

@@ -108,10 +113,12 @@ SECTIONS
   . = BSS_START;
   __bss_start = .;
   .bss			: { *(.bss) }
+  . = ALIGN(8);		/* the stack must be 64-bit aligned and adjoin bss */
   _end = .;
 
-  . = ALIGN(8);		/* the stack must be 64-bit aligned */
+  _stack_start = .;
   .stack		: { *(.stack) }
+  _stack_end = .;
 
   PROVIDE(__pecoff_data_size = ALIGN(512) - ADDR(.data));
   PROVIDE(__pecoff_end = ALIGN(512));

diff --git a/arch/arm/include/asm/image.h b/arch/arm/include/asm/image.h
index d5c18a0f6a34..624438740f23 100644
--- a/arch/arm/include/asm/image.h
+++ b/arch/arm/include/asm/image.h

@@ -15,6 +15,7 @@
 #define ARM_ZIMAGE_MAGIC1 ZIMAGE_MAGIC(0x016f2818)
 #define ARM_ZIMAGE_MAGIC2 (0x45454545)
 #define ARM_ZIMAGE_MAGIC3 ZIMAGE_MAGIC(0x5a534c4b)
+#define ARM_ZIMAGE_MAGIC4 ZIMAGE_MAGIC(0x5a534344)
 
 #ifndef __ASSEMBLY__
 

@@ -43,6 +44,18 @@ struct arm_zimage_tag {
 	} u;
 };
 
+struct arm_zimage_tag_dc {
+	struct tag_header hdr;
+	union {
+#define ZIMAGE_TAG_DECOMP_SIZE ARM_ZIMAGE_MAGIC4
+		struct zimage_decomp_size {
+			__le32 bss_size;
+			__le32 stack_size;
+			__le32 malloc_size;
+		} decomp_size;
+	} u;
+};
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* __ASM_IMAGE_H */

-- 
2.26.2

-- 
Łukasz Stelmach
Samsung R&D Institute Poland
Samsung Electronics