Re: [qemu-s390x] Interactive Boot Menu: New Fields in IPLB

[David Hildenbrand]

On 08.12.2017 00:56, Collin L. Walling wrote:
> Hello,
> 
> I'd like to bring up a formal discussion regarding the utilization of 
> reserved space
> in the IPL Parameter Blocks for s390x guests. Particularly the following:
> 
>   * How should we approach "claiming" bytes so that we do not obstruct
>     future development?
>   * What would be "the best" location to store these new fields?
> 
> I've posted some relevant information regarding this feature below.
> 
> 
>                        --- The Fields We Need To Store ---
> 
> Currently, we're utilizing 3 bytes out of a 77 byte reserved field in 
> the IPLB for
> CCW and SCSI. We do this because need a way to pass the Qemu command 
> line options
> for a boot menu (on/off and timeout) to the s390-ccw bios. We require 
> only two
> fields in the IPLB, totaling 3 bytes:
> 
>   * uint8_t boot_menu_flag
>       o determines if we should show the menu or not
> 
>   * uint16_t boot_menu_timeout
>       o stored as milliseconds
>       o A max value of approx 65,000 gives us 65 seconds -- should be 

1. Why on earth milliseconds. I'd rather have it remain open for 5
minutes than 1ms. 1s should really be a sufficient granularity.

2. Why not combine everything in one field?

0x00000000 -> Don't show
0x00000001 -> Show, wait 1s
...
0xffff0000 .. 0xfffffffe -> Reserved
0xffffffff -> Show, wait forever

We could add another special value for "use defaults from zipl"

-- 

Thanks,

David / dhildenb