Re: [kvm-unit-tests PATCH 1/5] s390x: Add a linker script to assembly snippets

[Janis Schoetterl-Glausch <scgl@linux.ibm.com>]

On Wed, 2022-11-23 at 08:46 +0000, Janosch Frank wrote:
> A linker script has a few benefits:
> - Random data doesn't end up in the binary breaking tests
> - We can easily define a lowcore and load the snippet from 0x0 instead
> of 0x4000 which makes asm snippets behave like c snippets
> - We can easily define an invalid PGM new PSW to ensure an exit on a
> guest PGM
> 
> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
> ---
>  lib/s390x/snippet.h         |  3 +--
>  s390x/Makefile              |  5 +++--
>  s390x/mvpg-sie.c            |  2 +-
>  s390x/pv-diags.c            |  6 +++---
>  s390x/snippets/asm/flat.lds | 43 +++++++++++++++++++++++++++++++++++++
>  5 files changed, 51 insertions(+), 8 deletions(-)
>  create mode 100644 s390x/snippets/asm/flat.lds
> 
> diff --git a/lib/s390x/snippet.h b/lib/s390x/snippet.h
> index b17b2a4c..57045994 100644
> --- a/lib/s390x/snippet.h
> +++ b/lib/s390x/snippet.h
> @@ -32,8 +32,7 @@
> 
>  #define SNIPPET_PV_TWEAK0	0x42UL
>  #define SNIPPET_PV_TWEAK1	0UL
> -#define SNIPPET_OFF_C		0
> -#define SNIPPET_OFF_ASM		0x4000
> +#define SNIPPET_UNPACK_OFF	0

You could also get rid of the offset parameter, couldn't you?
> 
> 
>  /*
> diff --git a/s390x/Makefile b/s390x/Makefile
> index bf1504f9..bb0f9eb8 100644
> --- a/s390x/Makefile
> +++ b/s390x/Makefile
> @@ -135,7 +135,8 @@ $(SNIPPET_DIR)/c/%.o: $(SNIPPET_DIR)/c/%.c $(asm-offsets)
>  	$(CC) $(CFLAGS) -c -nostdlib -o $@ $<
> 
>  $(SNIPPET_DIR)/asm/%.gbin: $(SNIPPET_DIR)/asm/%.o
> -	$(OBJCOPY) -O binary -j ".rodata" -j ".text" -j ".data" -j ".bss" --set-section-flags .bss=alloc,load,contents $(patsubst %.gbin,%.o,$@) $@
> +	$(CC) $(LDFLAGS) -o $@ -T $(SRCDIR)/s390x/snippets/asm/flat.lds $(patsubst %.gbin,%.o,$@)
> +	$(OBJCOPY) -O binary -j ".rodata" -j ".lowcore" -j ".text" -j ".data" -j ".bss" --set-section-flags .bss=alloc,load,contents $@ $@

I assume .bss=alloc allocates the bss in the binary...

>  	truncate -s '%4096' $@
> 
>  $(SNIPPET_DIR)/c/%.gbin: $(SNIPPET_DIR)/c/%.o $(snippet_lib) $(FLATLIBS)
> @@ -144,7 +145,7 @@ $(SNIPPET_DIR)/c/%.gbin: $(SNIPPET_DIR)/c/%.o $(snippet_lib) $(FLATLIBS)
>  	truncate -s '%4096' $@
> 
>  $(SNIPPET_DIR)/asm/%.hdr: $(SNIPPET_DIR)/asm/%.gbin $(HOST_KEY_DOCUMENT)
> -	$(GEN_SE_HEADER) -k $(HOST_KEY_DOCUMENT) -c $<,0x4000,0x00000000000000420000000000000000 --psw-addr 0x4000 -o $@
> +	$(GEN_SE_HEADER) -k $(HOST_KEY_DOCUMENT) -c $<,0x0,0x00000000000000420000000000000000 --psw-addr 0x4000 -o $@
> 
>  $(SNIPPET_DIR)/c/%.hdr: $(SNIPPET_DIR)/c/%.gbin $(HOST_KEY_DOCUMENT)
>  	$(GEN_SE_HEADER) -k $(HOST_KEY_DOCUMENT) -c $<,0x0,0x00000000000000420000000000000000 --psw-addr 0x4000 -o $@
> 
[...]

> diff --git a/s390x/snippets/asm/flat.lds b/s390x/snippets/asm/flat.lds
> new file mode 100644
> index 00000000..366d2d78
> --- /dev/null
> +++ b/s390x/snippets/asm/flat.lds
> @@ -0,0 +1,43 @@
> +SECTIONS
> +{
> +	.lowcore : {
> +		/*
> +		 * Initial short psw for disk boot, with 31 bit addressing for
> +		 * non z/Arch environment compatibility and the instruction
> +		 * address 0x4000.
> +		 */
> +		. = 0;
> +		 LONG(0x00080000)
> +		 LONG(0x80004000)
> +		 /* Restart new PSW for booting via PSW restart. */
> +		 . = 0x1a0;
> +		 QUAD(0x0000000180000000)
> +		 QUAD(0x0000000000004000)
> +		 /*
> +		  * Invalid PGM new PSW so we hopefully get a code 8
> +		  * intercept on a PGM
> +		  */
> +		 . = 0x1d0;
> +		 QUAD(0x0008000000000000)
> +		 QUAD(0x0000000000000001)
> +	}
> +	. = 0x4000;
> +	.text : {
> +		*(.text)
> +		*(.text.*)
> +	}
> +	. = ALIGN(64K);
> +	etext = .;
> +	. = ALIGN(16);
> +	.data : {
> +		*(.data)
> +		*(.data.rel*)
> +	}
> +	. = ALIGN(16);
> +	.rodata : { *(.rodata) *(.rodata.*) }
> +	. = ALIGN(16);
> +	__bss_start = .;

.. so the __bss symbols are not necessary.
But then, the c flat.lds has them too.
> +	.bss : { *(.bss) }
> +	__bss_end = .;
> +	. = ALIGN(64K);
> +}