[coreboot] [RFC] SMM handling and resident coreboot

Stefan Reinauer stepan at coresystems.de
Mon Jul 28 14:11:35 CEST 2008

Carl-Daniel Hailfinger wrote:
>> Another alternative to keeping full coreboot around, would be to make
>> the SMM handler self contained. This would mean, the SMM handler could
>> not use coreboot's functions like printk_debug, pci_read_config32, it
>> could not use the device tree, and it would become more complex, because
>> for some information we have to reprobe the hardware, or parse the
>> coreboot table.
> From a v3 perspective, it might make sense to keep the SMI handler (if
> it has limited size) in the boot block. That avoids any and all
> requirements to keep code in RAM, thereby taking away the problem of
> code relocation.

I thought about this, too, but unfortunately it won't work. Two problems:

* This still does not solve the problem for ACPI, and the coreboot
table. That they survive is currently pure luck, and this is version
agnostic, and starts causing trouble as soon as we think about scenarios
like cleanly incorporating seabios.
* There is no heap nor data nor bss available, so we keep all
disadvantages of keeping SMI self contained, except code duplication of
maybe printk_debug() and pci_[read|write]_config[8|16|32] and/or
possibly their enhanced PCIe versions. Considering that in a productive
system printk falls away, making the SMM handler self contained is a
considerable option.
>> In the case of the SMM handler, this would also confine us, because the
>> actual SMI# handling code (written in C) would not be shared between
>> CPUs but has to be duplicated for every CPU core. However, my current
>> approach only keeps a very small amount of code per CPU, that is just
>> enough to enter gcc compiled functions and return from them, cleanly.
> AFAIK factory BIOS SMM handlers have the ability to lock down the memory
> segment they are using, protecting them from accidental or deliberate
> tampering by the OS (which could lead to interesting security issues).
> Can we do that even if the handler is "somewhere" in RAM?
Not really. There are certain memory regions that can be locked. But the
smi_handler() C function does not live there, currently. If we want to
lock out the OS, we need to go down the route of being self-contained.

>> One of the questions in my mind is: where should we put the coreboot
>> image, if we want to keep it around?
>> A little excerpt from coreboot v2:
>> I know the problem of where to put coreboot has been thought about
>> before, elfboot() relocates coreboot to another place when loading an
>> ELF binary that demands the space where coreboot lives:
>>  * coreboot tries to load a segment and finds out, that it is in the way.
>>  * coreboot copies itself to a new position
>>  * coreboot jumps into the assembler handler in jmp_to_elf_entry at the
>> new position
>>  * coreboot tries to start the ELF binary.
>>  * If it fails, it overwrites the loaded ELF binary by copying itself
>> back and jumping to the original position.
>> This is quite an interesting concept, but it also makes clear that the
>> ram portion of coreboot itself ("stage2") can not be relocated freely in
>> memory. Yet.
> Please see my other mail in this thread about possible problems with a
> relocatable stage2. Besides that, we'd need a way in v3 to tag a LAR
> member as PIC (sort of done for the special case of XIP in initram) and
> new code to figure out a good load address during run time.
So you are saying we can't make stage2 relocatable because noone else
did before, or because we would have to write new code? I think both are
acceptable risks given that v3 has a user base of exactly zero and is
not in the shape to carry a port to any non-embedded systems anyways.

>> Since we know how big our RAM is when we copy coreboot to RAM, I suggest
>> that we copy coreboot to the end of memory and run it from there. It is
>> a pretty good assumption that no payload will require that space. During
>> memory map creation, we just reserve 256k at the upper end, and we're good.  
> Hm... I assume end of memory is "end of memory below 4G". If we want to
> avoid conflicts with mapped memory areas of extension cards, stage2 code
> has to be loaded twice: Once before setting up extension cards (load
> stage2 to above 1M or other failsafe location) and after setting up
> extension cards (load stage2 to end of memory below 4G and below
> extension card space). That's not exactly the nicest code flow I can
> think of.

Yes, I guess I was referring to anything that fits in 32bit address
space, as there is no coreboot port utilizing 64bit addressing modes.
End of memory is end of _memory_, not end of address space. The memory
hole for PCI devices is hard coded in coreboot, and most chipsets allow
to remap memory "under" the hole to above 4G.


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