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

On 11/27/23 2:46 PM, Warner Losh wrote:
> The branch main has been updated by imp:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=c596126a5d3d2ee015ee6807b4041efa5b9d9b07
> 
> commit c596126a5d3d2ee015ee6807b4041efa5b9d9b07
> Author:     Warner Losh <imp@FreeBSD.org>
> AuthorDate: 2023-11-27 22:40:40 +0000
> Commit:     Warner Losh <imp@FreeBSD.org>
> CommitDate: 2023-11-27 22:45:56 +0000
> 
>      pmbr: Only load the first 545k rather than error out
>      
>      It would be nice to have larger boot partitions for ESPs to live in one
>      day. It's trivial to carve out 5M 10M or 200M when provisioning, but
>      logistical issues may make it hard to do it after the fact. So only warn
>      when the partition is > 545k. If we ever grow the boot loader larger
>      than that, then it will be responsible for loading the rest anyway.
>      
>      Sponsored by:           Netflix
>      Reviewed by:            tsoome
>      Differential Revision:  https://reviews.freebsd.org/D42774
> ---
>   stand/i386/pmbr/pmbr.S | 24 +++++++++++++++---------
>   1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/stand/i386/pmbr/pmbr.S b/stand/i386/pmbr/pmbr.S
> index c61acbe261bd..60b26df15720 100644
> --- a/stand/i386/pmbr/pmbr.S
> +++ b/stand/i386/pmbr/pmbr.S
> @@ -155,8 +155,10 @@ next_boot:	addl $1,(%si)			# Next LBA
>   		mov %es,%ax			# Adjust segment for next
>   		addw $SECSIZE/16,%ax		#  sector
>   		cmp $0x9000,%ax			# Don't load past 0x90000,
> -		jae err_big			#  545k should be enough for
> -		mov %ax,%es			#  any boot code. :)
> +		jb sz_ok			#  545k should be enough for
> +		call err_big			#  any boot code, but warn
> +		mov $0x9000-SECSIZE/16,%ax	#  and truncate
> +sz_ok:		mov %ax,%es
>   		jmp load_boot
>   #
>   # Move to the next partition.  If we walk off the end of the sector, load
> @@ -203,17 +205,21 @@ getdrvparams:
>   #
>   # Various error message entry points.
>   #
> -err_big: 	movw $msg_big,%si		# "Boot loader too
> -		jmp putstr			#  large"
> +err_big: 	movw $msg_big,%si		# "Truncated
> +		call putstr			#  to 545k"
> +		ret

Hmm, the comment here and the text string don't match.

>   err_pt: 	movw $msg_pt,%si		# "Invalid partition
> -		jmp putstr			#  table"
> +		call putstr			#  table"
> +err_pt.1:	jmp err_pt.1			# Await reset
>   
>   err_rd: 	movw $msg_rd,%si		# "I/O error loading
> -		jmp putstr			#  boot loader"
> +		call putstr			#  boot loader"
> +		jmp err_pt.1
>   
>   err_noboot: 	movw $msg_noboot,%si		# "Missing boot
> -		jmp putstr			#  loader"
> +		call putstr			#  loader"
> +		jmp err_pt.1
>   #
>   # Output an ASCIZ string to the console via the BIOS.
>   #
> @@ -223,9 +229,9 @@ putstr.0:	movw $0x7,%bx	 		# Page:attribute
>   putstr: 	lodsb				# Get character
>   		testb %al,%al			# End of string?
>   		jnz putstr.0			# No
> -putstr.1:	jmp putstr.1			# Await reset
> +		ret
>   
> -msg_big: 	.asciz "Boot loader too large"
> +msg_big: 	.asciz "Loaded only 545k"
>   msg_pt: 	.asciz "Invalid partition table"
>   msg_rd: 	.asciz "I/O error loading boot loader"
>   msg_noboot: 	.asciz "Missing boot loader"

I wonder if it would save a few bytes by having a common 'puterr'
that does 'call putstr' and then the jmp to itself that the other err
targets use rahter than adding the jmp to each one of those.

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.

-- 
John Baldwin