Re: [PATCH 2/3] OvmfPkg: introduce FD_SIZE_4MB (mainly) for Windows HCK

[Laszlo Ersek <lersek@redhat.com>]

On 04/30/17 02:48, Jordan Justen wrote:

> Today, we give 128k to non-volatile storage, which due to various
> reasons results in 56k of variable storage. Personally I already
> consider this generous, especially when I look at the 128k size of the
> seabios bios.bin. :)

I'd like to comment on the seabios analogy; I just needed a bit more
time to gather data for this (BZs and build logs, mainly).

In RHEL-6, we shipped (and have been shipping) 128KB SeaBIOS binaries.

Preparing for RHEL-7.0, we increased the SeaBIOS binary size to 256KB,
anticipating future features that would no longer fit into 128KB. This
is the BZ for it:

https://bugzilla.redhat.com/show_bug.cgi?id=1038604

The exact "seabios-1.7.2.2-5.el7" build, dated 2013-Dec-10, in which the
BZ was addressed, is no longer available in our build archives (it was
not a released version, just a development build, so that's OK), but
"seabios-1.7.2.2-6.el7", i.e. the build right after, is available. At
that time, we were still under 128KB:

> Total size: 117472  Fixed: 58792  Free: 144672
> (used 44.8% of 256KiB rom)

Today, looking at the build log of "seabios-1.10.2-2.el7" (built
2017-Mar-30), I find:

> Total size: 149664  Fixed: 75392  Free: 112480
> (used 57.1% of 256KiB rom)

Had we not increased the size in advance (via CONFIG_ROM_SIZE), we'd
have run out of it over the past ~3.5 years.


In OVMF, the address range of the non-volatile area depends on SECFV's
size, FVMAIN_COMPACT's size, and on the NV area's size; they come in
this order downwards from 4GB.

SECFV is safe as-is; it has seen negligible growth in years, and is
currently at about 14% in a DEBUG build.

I'd like to future-proof the other two (FVMAIN_COMPACT and NV) sizes
now, with a large -- approx. seven year -- margin.

Thanks
Laszlo