Re: [Qemu-devel] [PATCH 3/3] s390: Do not pass inofficial IPL type to the guest

[David Hildenbrand <david@redhat.com>]

On 05.04.2018 17:07, Viktor Mihajlovski wrote:
> IPL over a virtio-scsi device requires special handling not
> available in the real architecture. For this purpose the IPL
> type 0xFF has been chosen as means of communication between
> QEMU and the pc-bios. However, a guest OS could be confused
> by seeing an unknown IPL type.
> 
> This change sets the IPL parameter type to 0x02 (CCW) to prevent
> this. Pre-existing Linux has looked up the IPL parameters only in
> the case of FCP IPL. This means that the behavior should stay
> the same even if Linux checks for the IPL type unconditionally.
> 
> Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>
> ---
>  pc-bios/s390-ccw/bootmap.c |  7 +++++++
>  pc-bios/s390-ccw/iplb.h    | 15 +++++++++++++--
>  2 files changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/pc-bios/s390-ccw/bootmap.c b/pc-bios/s390-ccw/bootmap.c
> index fc2a9fe..9287b7a 100644
> --- a/pc-bios/s390-ccw/bootmap.c
> +++ b/pc-bios/s390-ccw/bootmap.c
> @@ -70,6 +70,13 @@ static void jump_to_IPL_code(uint64_t address)
>  {
>      /* store the subsystem information _after_ the bootmap was loaded */
>      write_subsystem_identification();
> +
> +    /* prevent unknown IPL types in the guest */
> +    if (iplb.pbt == S390_IPL_TYPE_QEMU_SCSI) {
> +        iplb.pbt = S390_IPL_TYPE_CCW;
> +        set_iplb(&iplb);
> +    }

Confused, doesn't this imply that a system reset immediately after this
instruction will result in something different getting booted?

> +
>      /*
>       * The IPL PSW is at address 0. We also must not overwrite the
>       * content of non-BIOS memory after we loaded the guest, so we
> diff --git a/pc-bios/s390-ccw/iplb.h b/pc-bios/s390-ccw/iplb.h
> index 7dfce4f..5357a36 100644
> --- a/pc-bios/s390-ccw/iplb.h
> +++ b/pc-bios/s390-ccw/iplb.h
> @@ -97,16 +97,27 @@ extern QemuIplParameters qipl;
>  #define S390_IPL_TYPE_CCW 0x02
>  #define S390_IPL_TYPE_QEMU_SCSI 0xff
>  
> -static inline bool store_iplb(IplParameterBlock *iplb)
> +static inline bool manage_iplb(IplParameterBlock *iplb, bool store)
>  {
>      register unsigned long addr asm("0") = (unsigned long) iplb;
>      register unsigned long rc asm("1") = 0;
>  
>      asm volatile ("diag %0,%2,0x308\n"
>                    : "+d" (addr), "+d" (rc)
> -                  : "d" (6)
> +                  : "d" (store ? 6 : 5)
>                    : "memory", "cc");
>      return rc == 0x01;
>  }
>  
> +
> +static inline bool store_iplb(IplParameterBlock *iplb)
> +{
> +    return manage_iplb(iplb, true);
> +}
> +
> +static inline bool set_iplb(IplParameterBlock *iplb)
> +{
> +    return manage_iplb(iplb, false);
> +}
> +
>  #endif /* IPLB_H */
> 


-- 

Thanks,

David / dhildenb