Re: [kvm-unit-tests PATCH v4 4/6] s390x: Add library functions for exiting from snippet

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

On 10/16/24 8:03 PM, Nina Schoetterl-Glausch wrote:
> It is useful to be able to force an exit to the host from the snippet,
> as well as do so while returning a value.
> Add this functionality, also add helper functions for the host to check
> for an exit and get or check the value.
> Use diag 0x44 and 0x9c for this.
> Add a guest specific snippet header file and rename snippet.h to reflect
> that it is host specific.
> 
> Signed-off-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
> ---
>   s390x/Makefile                    |  4 ++-
>   lib/s390x/asm/arch_def.h          | 13 +++++++++
>   lib/s390x/snippet-exit.h          | 47 +++++++++++++++++++++++++++++++
>   s390x/snippets/lib/snippet-exit.h | 28 ++++++++++++++++++
>   4 files changed, 91 insertions(+), 1 deletion(-)
>   create mode 100644 lib/s390x/snippet-exit.h
>   create mode 100644 s390x/snippets/lib/snippet-exit.h
> 
> diff --git a/s390x/Makefile b/s390x/Makefile
> index 0ad8d021..1caf221d 100644
> --- a/s390x/Makefile
> +++ b/s390x/Makefile
> @@ -70,7 +70,8 @@ test_cases: $(tests)
>   test_cases_binary: $(tests_binary)
>   test_cases_pv: $(tests_pv_binary)
>   
> -INCLUDE_PATHS = $(SRCDIR)/lib $(SRCDIR)/lib/s390x $(SRCDIR)/s390x
> +SNIPPET_INCLUDE :=
> +INCLUDE_PATHS = $(SNIPPET_INCLUDE) $(SRCDIR)/lib $(SRCDIR)/lib/s390x $(SRCDIR)/s390x
>   # Include generated header files (e.g. in case of out-of-source builds)
>   INCLUDE_PATHS += lib
>   CPPFLAGS = $(addprefix -I,$(INCLUDE_PATHS))
> @@ -151,6 +152,7 @@ endif
>   $(SNIPPET_DIR)/asm/%.o: $(SNIPPET_DIR)/asm/%.S $(asm-offsets)
>   	$(CC) $(CFLAGS) -c -nostdlib -o $@ $<
>   
> +$(SNIPPET_DIR)/c/%.o: SNIPPET_INCLUDE := $(SNIPPET_DIR)/lib
>   $(SNIPPET_DIR)/c/%.o: $(SNIPPET_DIR)/c/%.c $(asm-offsets)
>   	$(CC) $(CFLAGS) -c -nostdlib -o $@ $<
>   
> diff --git a/lib/s390x/asm/arch_def.h b/lib/s390x/asm/arch_def.h
> index 745a3387..db04deca 100644
> --- a/lib/s390x/asm/arch_def.h
> +++ b/lib/s390x/asm/arch_def.h
> @@ -504,4 +504,17 @@ static inline uint32_t get_prefix(void)
>   	return current_prefix;
>   }
>   
> +static inline void diag44(void)
> +{
> +	asm volatile("diag	0,0,0x44\n");
> +}
> +
> +static inline void diag9c(uint64_t val)
> +{
> +	asm volatile("diag	%[val],0,0x9c\n"
> +		:
> +		: [val] "d"(val)
> +	);
> +}
> +
>   #endif
> diff --git a/lib/s390x/snippet-exit.h b/lib/s390x/snippet-exit.h
> new file mode 100644
> index 00000000..f62f0068
> --- /dev/null
> +++ b/lib/s390x/snippet-exit.h
> @@ -0,0 +1,47 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Functionality handling snippet exits
> + *
> + * Copyright IBM Corp. 2024
> + */
> +
> +#ifndef _S390X_SNIPPET_EXIT_H_
> +#define _S390X_SNIPPET_EXIT_H_
> +
> +#include <libcflat.h>
> +#include <sie.h>
> +#include <sie-icpt.h>
> +
> +static inline bool snippet_is_force_exit(struct vm *vm)
> +{
> +	return sie_is_diag_icpt(vm, 0x44);
> +}
> +
> +static inline bool snippet_is_force_exit_value(struct vm *vm)
> +{
> +	return sie_is_diag_icpt(vm, 0x9c);
> +}
> +
> +static inline uint64_t snippet_get_force_exit_value(struct vm *vm)
> +{
> +	struct kvm_s390_sie_block *sblk = vm->sblk;
> +
> +	assert(snippet_is_force_exit_value(vm));
> +
> +	return vm->save_area.guest.grs[sblk_ip_as_diag(sblk).r_1];
> +}

The cpu address parameter for 9C is 16 bit.
While we could make it 64 bit for snippets I don't see a reason to do 
so. The 16 bits are enough to indicate something to the host which can 
then go and fetch memory for more data.