[SeaBIOS] [PATCH] Revert "mptable: Don't describe pci-to-pci bridges."

[Liran Alon]

> On 2 Dec 2018, at 18:31, Kevin O'Connor <kevin at koconnor.net> wrote:
> 
> On Fri, Nov 30, 2018 at 09:53:05PM +0200, Liran Alon wrote:
>>> On 30 Nov 2018, at 21:25, Kevin O'Connor <kevin at koconnor.net> wrote:
>>> These tables are considered static in SeaBIOS.  We've found that any
>>> change to these legacy tables tends to break something.  We moved the
>>> generation of SMBIOS and ACPI to QEMU for this reason.  I don't think
>>> MPTable was moved into QEMU, but I think that should be the way
>>> forward if a particular guest needs different content in this table.
>> 
>> Makes sense.
>> 
>> Until that MPTable will be propagated to SeaBIOS via fw_cfg from QEMU, don’t you think it makes sense to revert this commit for now?
>> As it caused a real guest workload to break (with non-trivial issue to diagnose) and doesn’t provide a real value of it’s own (or am I missing something?).
>> 
> 
> Hi Liran,
> 
> The original commit is from almost six years ago.  I know it's very
> difficult to track down these types of regressions.  I don't doubt it
> took a large effort to find the root cause.  However, there's a very
> real risk that a change (any change) would introduce a further
> regression.
> 
> Even when the change is correct, we know from experience that
> different operating systems sometimes take unexpected actions when
> parsing these tables.  In particular, we've seen in the past that
> innocuous changes to the bios tables can cause some operating systems
> to redo "hardware scans".  For these (and other) reasons we've moved
> bios table generation out of SeaBIOS.
> 
> So, I don't think it would be a good idea to make this change to
> SeaBIOS.
> 
> Thanks,
> -Kevin

Hmm OK. I understand your point.
BTW, we have run a big variety of OSes with this change and haven’t seen a regression.

So you recommend that even if we implement the mechanism to pass mptable via fw_cfg from QEMU that this specific
behaviour will be controlled by a flag? That is doable. Just wondering on your opinion.

-Liran