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

[Janosch Frank <frankja@linux.ibm.com>]

On 11/24/22 21:42, Janis Schoetterl-Glausch wrote:
> 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?

Right

>>
>>
>>   /*
>> 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...

And that's fine since I don't want to handle 0x3E PGMs/faults.

If bss is in the binary then it'll be made secure on initial image 
unpack, if it isn't, then we need a handler to import pages on a 0x3E. 
And I don't really want to do that since a 0x3E could also mean that we 
have an issue with the test or HW/FW.

> 
>>   	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);
>> +}
>