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

Janosch Frank
2022-11-24T14:28:57Z

On 11/23/22 13:45, Claudio Imbrenda wrote:
> On Wed, 23 Nov 2022 08:46:52 +0000
> Janosch Frank <frankja@linux.ibm.com> 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>
>
> looks good in general, but I have a few questions
>
>> ---
>> 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
>>
>>
>> /*
>> 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 $(patsubst %.gbin,%.o,$@)
>
> I think you can simply use $< instead of the patsubst expression

Right, I'll fix that momentarily.

[...]

>> + .text : {
>> + *(.text)
>> + *(.text.*)
>> + }
>> + . = ALIGN(64K);
>
> any reason to align to 64k? (instead of e.g. 4k)

Well, I've copied that from s390x/flat.lds...
I'll have a look at the required alignments when I find time.

>
>> + etext = .;
>> + . = ALIGN(16);
>
> do you need the ALIGN? I would think we are already aligned here
>
>> + .data : {
>> + *(.data)
>> + *(.data.rel*)
>> + }
>> + . = ALIGN(16);
>> + .rodata : { *(.rodata) *(.rodata.*) }
>> + . = ALIGN(16);
>> + __bss_start = .;
>> + .bss : { *(.bss) }
>> + __bss_end = .;
>> + . = ALIGN(64K);
>
> same question as above regarding 64k
>
>> +}
>