Re: [PATCH v2 2/4] pc-bios: s390x: Use reset PSW if avaliable

[Thomas Huth]

On 27/08/2020 11.31, Janosch Frank wrote:
> If a blob provides a reset PSW then we should use it instead of
> branching to the PSW address and using our own mask.
> 
> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
> ---
>  pc-bios/s390-ccw/bootmap.c  |  1 +
>  pc-bios/s390-ccw/jump2ipl.c | 48 ++++++++++++++++++++++---------------
>  pc-bios/s390-ccw/s390-ccw.h |  1 +
>  3 files changed, 31 insertions(+), 19 deletions(-)
> 
> diff --git a/pc-bios/s390-ccw/bootmap.c b/pc-bios/s390-ccw/bootmap.c
> index 8747c4ea26..0df9b3781d 100644
> --- a/pc-bios/s390-ccw/bootmap.c
> +++ b/pc-bios/s390-ccw/bootmap.c
> @@ -515,6 +515,7 @@ static void zipl_run(ScsiBlockPtr *pte)
>      IPL_assert(entry->component_type == ZIPL_COMP_ENTRY_EXEC, "No EXEC 
> entry");
>  
>      /* should not return */
> +    write_reset_psw(entry->compdat.load_psw);
>      jump_to_IPL_code(entry->compdat.load_psw & PSW_MASK_SHORT_ADDR);

Shouldn't that be jump_to_IPL_code(0) now? Wouldn't it be cleaner to
have a jump_to_IPL_PSW() function instead?

>  }
>  
> diff --git a/pc-bios/s390-ccw/jump2ipl.c b/pc-bios/s390-ccw/jump2ipl.c
> index 767012bf0c..143d027bf7 100644
> --- a/pc-bios/s390-ccw/jump2ipl.c
> +++ b/pc-bios/s390-ccw/jump2ipl.c
> @@ -13,20 +13,28 @@
>  #define KERN_IMAGE_START 0x010000UL
>  #define RESET_PSW_MASK (PSW_MASK_SHORTPSW | PSW_MASK_64)
>  
> -typedef struct ResetInfo {
> -    uint64_t ipl_psw;
> -    uint32_t ipl_continue;
> -} ResetInfo;
> -
> -static ResetInfo save;
> +uint64_t *reset_psw = 0, save_psw, ipl_continue;

I think the patch would be better readable if you'd split it in two -
first the ResetInfo rework, then the use-reset-PSW-if-available stuff ?

>  static void jump_to_IPL_2(void)
>  {
> -    ResetInfo *current = 0;
> +    /* Restore reset PSW and io and external new PSWs */
> +    *reset_psw = save_psw;

The comment talks about io and external new PSWs ... but where is it in
the code?

> -    void (*ipl)(void) = (void *) (uint64_t) current->ipl_continue;
> -    *current = save;
> -    ipl(); /* should not return */
> +    /* No reset PSW, let's jump instead. */
> +    if (ipl_continue) {
> +        void (*ipl)(void) = (void *) (uint64_t) ipl_continue;

Is it possible to mark a function pointer with __attribute__((noreturn)) ?

> +        ipl();
> +    }
> +
> +    /* Reset PSW available, let's load it */
> +    asm volatile ("lpsw 0(%0)\n"
> +        :  : "a" (0):);

I've never tried. but maybe you could add __attribute__((noreturn)) to
an inline asm statement, too?

> +    /* should not return */
> +}
> +
> +void write_reset_psw(uint64_t psw)
> +{
> +    *reset_psw = psw;
>  }
>  
>  void jump_to_IPL_code(uint64_t address)
> @@ -46,15 +54,12 @@ void jump_to_IPL_code(uint64_t address)
>       * content of non-BIOS memory after we loaded the guest, so we
>       * save the original content and restore it in jump_to_IPL_2.
>       */
> -    ResetInfo *current = 0;
> +    save_psw = *reset_psw;
> +    *reset_psw = (uint64_t) &jump_to_IPL_2;
> +    *reset_psw |= RESET_PSW_MASK;
> +    ipl_continue = address;
>  
> -    save = *current;
> -
> -    current->ipl_psw = (uint64_t) &jump_to_IPL_2;
> -    current->ipl_psw |= RESET_PSW_MASK;
> -    current->ipl_continue = address & PSW_MASK_SHORT_ADDR;
> -
> -    debug_print_int("set IPL addr to", current->ipl_continue);
> +    debug_print_int("set IPL addr to", ipl_continue);
>  
>      /* Ensure the guest output starts fresh */
>      sclp_print("\n");
> @@ -84,7 +89,12 @@ void jump_to_low_kernel(void)
>  
>      /* Trying to get PSW at zero address */
>      if (*((uint64_t *)0) & RESET_PSW_MASK) {
> -        jump_to_IPL_code((*((uint64_t *)0)) & PSW_MASK_SHORT_ADDR);
> +        /*
> +         * Surely nobody will try running directly from lowcore, so
> +         * let's use 0 as an indication that we want to load the reset
> +         * psw at 0x0 and not jump to the entry.
> +         */
> +        jump_to_IPL_code(0);
>      }
>  
>      /* No other option left, so use the Linux kernel start address */
> diff --git a/pc-bios/s390-ccw/s390-ccw.h b/pc-bios/s390-ccw/s390-ccw.h
> index 36b884cced..7090720422 100644
> --- a/pc-bios/s390-ccw/s390-ccw.h
> +++ b/pc-bios/s390-ccw/s390-ccw.h
> @@ -78,6 +78,7 @@ int virtio_read(ulong sector, void *load_addr);
>  void zipl_load(void);
>  
>  /* jump2ipl.c */
> +void write_reset_psw(uint64_t psw);
>  void jump_to_IPL_code(uint64_t address);
>  void jump_to_low_kernel(void);
>  
> 

 Thomas