Re: git: c596126a5d3d - main - pmbr: Only load the first 545k rather than error out

On 11/28/23 9:07 AM, Warner Losh wrote:
> On Tue, Nov 28, 2023 at 9:06 AM John Baldwin <jhb@freebsd.org> wrote:
>> But also, can't you just leave a gap in the partitions to leave room
>> for a future ESP?  It might not always be safe to load a full 545k
>> depending on if the firmware has reserved some of memory just below
>> 640k.  545k is an upper bound on how much can be loaded in terms of
>> 640k - the load address, but the practical limit might be lower.  It is
>> probably safer as a general rule to keep the boot partition closer to
>> the size of the  boot loader and leave a gap in place instead.
>>
> 
> Today we load 512k always.

Well, we load the partition size always.  The default size of the partition
has grown over time.  Arguably we should have been using padding instead of
growing the partition, but that is harder to write in installer tools, etc.
When I wrote pmbr I on purpose used the partition size as the size to load
rather than trying to assume information about the payload and parse a
length from the payload due to the limited room in the MBR.  I think we
started out creating freebsd-boot partitions that were 64k and later grew
them with a desire they could be replaced with an ESP partition, but ESPs
need a minimum size of 512k?

-- 
John Baldwin