Re: [kvm-unit-tests PATCH] s390x: Ignore gcc 12 warnings for low addresses

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Mon, 16 May 2022 16:43:32 +0200
Janis Schoetterl-Glausch <scgl@linux.ibm.com> wrote:

> gcc 12 warns if a memory operand to inline asm points to memory in the
> first 4k bytes. However, in our case, these operands are fine, either
> because we actually want to use that memory, or expect and handle the
> resulting exception.
> Therefore, silence the warning.

I really dislike this

> 
> Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
> ---
> 
> Alternatives:
>  * Use memory clobber instead of memory output
>    Use address in register input instead of memory input
>        (may require WRITE_ONCE)

this sounds better. would you also get rid of the asm("0") annotations
on the variables? I really dislike those too

>  * Disable the warning globally
>  * Don't use gcc 12.0, with newer versions --param=min-pagesize=0 might
>    avoid the problem
> 
>  lib/s390x/asm/cpacf.h | 7 +++++++
>  s390x/skey.c          | 7 +++++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/lib/s390x/asm/cpacf.h b/lib/s390x/asm/cpacf.h
> index 685262b0..02e603c8 100644
> --- a/lib/s390x/asm/cpacf.h
> +++ b/lib/s390x/asm/cpacf.h
> @@ -152,6 +152,12 @@ static __always_inline void __cpacf_query(unsigned int opcode, cpacf_mask_t *mas
>  	register unsigned long r0 asm("0") = 0;	/* query function */
>  	register unsigned long r1 asm("1") = (unsigned long) mask;
>  
> +/*
> + * gcc 12.0.1 warns if mask is < 4k.
> + * We use such addresses to test invalid or protected mask arguments.
> + */
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Warray-bounds"
>  	asm volatile(
>  		"	spm 0\n" /* pckmo doesn't change the cc */
>  		/* Parameter regs are ignored, but must be nonzero and unique */
> @@ -160,6 +166,7 @@ static __always_inline void __cpacf_query(unsigned int opcode, cpacf_mask_t *mas
>  		: "=m" (*mask)
>  		: [fc] "d" (r0), [pba] "a" (r1), [opc] "i" (opcode)
>  		: "cc");
> +#pragma GCC diagnostic pop
>  }
>  
>  static inline int __cpacf_check_opcode(unsigned int opcode)
> diff --git a/s390x/skey.c b/s390x/skey.c
> index 32bf1070..7aa91d19 100644
> --- a/s390x/skey.c
> +++ b/s390x/skey.c
> @@ -242,12 +242,19 @@ static void test_store_cpu_address(void)
>   */
>  static void set_prefix_key_1(uint32_t *prefix_ptr)
>  {
> +/*
> + * gcc 12.0.1 warns if prefix_ptr is < 4k.
> + * We need such addresses to test fetch protection override.
> + */
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Warray-bounds"
>  	asm volatile (
>  		"spka	0x10\n\t"
>  		"spx	%0\n\t"
>  		"spka	0\n"
>  	     :: "Q" (*prefix_ptr)
>  	);
> +#pragma GCC diagnostic pop
>  }
>  
>  /*
> 
> base-commit: c315f52b88b967cfb4cd58f3b4e1987378c47f3b