[SeaBIOS] Methods to reduce bios image size

[Paolo Bonzini]

On 24/03/2015 17:22, Bruce Rogers wrote:
> All,
> 
> When building SeaBIOS in the context of QEMU v2.3-rc0 using
> various openSUSE versions we found that some recent versions
> overflowed the 128K limit imposed for bios.bin. The Makefile
> fails with the suggestion of using a more recent toolset or of
> eliminating some features to make it fit. Neither of those are
> options in this case.

This is the Fedora config file for 128K:

CONFIG_QEMU=y
CONFIG_ROM_SIZE=128
CONFIG_XEN=n
CONFIG_USB_XHCI=n
CONFIG_USB_UAS=n

An alternative is to leave in XHCI and UAS, and disable CONFIG_PVSCSI,
CONFIG_ESP_SCSI and CONFIG_LSI_SCSI.

Paolo


> Of course another option is to review the code for places where
> variables or strings can be reduced in size without changing
> functionality. That doesn't buy us all that much, but enough
> for all of our cases except the oldest toolset. I'll post the
> patches I came up with during this review of the SeaBIOS code
> shortly to see what people think.
> 
> The 128K bios.bin needed by older QEMU machine types doesn't
> need new functionality, but some future SeaBIOS changes will
> undoubtedly still cause it to continue to grow a bit. Since we are
> right at the boundary of overflowing 128K, I thought it was worth
> investing some effort in backing away from that limit so we don't
> have to constantly worry about every small change causing an
> issue there.
> 
> In addition to the above mentioned data size reductions, another thing
> I noticed was that the e820 array is 32 entries, as given by BUILD_MAX_E820.
> It appears that QEMU only will ever fill 16 of those entries, so perhaps
> it would make sense to reduce that size when the usage is for QEMU only.
> 
> As a more general query, I was wondering if anyone else knew of places
> where the size of SeaBIOS could be reduced? I'd be happy to try to help
> make that happen.
> 
> Thanks,
> 
> Bruce Rogers
>