Re: [Patch 2/3] firmware: dmi_scan: add SBMIOS entry and DMI tables

[Jean Delvare]

Hi Roy,

On Wed, 15 Apr 2015 17:54:51 -0700, Roy Franz wrote:
> On Tue, Apr 14, 2015 at 9:19 PM, Roy Franz <roy.franz@xxxxxxxxxx> wrote:
> > I have made modifications to dmidecode to support this interface, and it
> > works quite nicely for dmidecode. (changes posted to dmidecode-devel list)
> > The only open issue I am aware of is how the SMBIOS v3 entry point
> > will be handled,
> > especially in cases where there is a v2 and a v3 entry point.
> > In principal I think this a good change - are there any other open issues?
> 
> I looked at the SMBIOS spec again, and the platform can provide either or
> both of the 32-bit and 64-bit entry points.  The spec says that an
> implementation
> "should" provide a 32-bit entry point for compatibility.
> 
> These 2 entry point structures can both point to the same SMBIOS
> structure table,
> or two distinct ones.  If distinct, the one referenced by the 32-bit
> entry point must be
> a consistent subset of the 64-bit one.
> There does not seem to be any prohibition from using new SMBIOS v3 structures
> in a table referenced by a 32-bit entry point, so backwards
> compatibility is completely
> up to the implementation.

What do you mean with "new SMBIOS v3 structures"? I took a brief look
at the change log at the end of the SMBIOS 3.0.0 specification, and all
I can see are new enumerated values for existing fields, as well as 3
new fields in the type 4 structure. I can't see any new structure type.
Either way this is all backwards compatible, which is why both entry
points can point to the same table.

> Since the point of this patchset (and related changes to dmidecode) is
> to provide the
> SMBIOS information without using /dev/mem, I think the interface we
> define should
> support all the cases outlined in the specification.  I would like to
> avoid a case where
> we're back to using /dev/mem to deal with the dual entry/dual table
> case if that becomes
> important down the road.
> 
> Here's a proposal for files in /sys/firmware/dmi/tables:
> 
> smbios_entry_point32   -  32 bit (existing entry point type), if it exists.
> smbios_entry_point64   -  64 bit entry, new in SMBIOS v3.0
> DMI32                         -  smbios structure tables referenced by
> 32 bit entry point, if it exists
> DMI64                         -  smbios structure tables referenced by
> 64 bit entry point, if it exists.
>                                      symlink to DMI32 if identical
> smbios_entry_point    - symlink to smbios_entry_point64 if it exists,
> otherwise symlink to smbios_entry_point64
> DMI                           - symlink to DMI64 if it exists,
> otherwise symlink to DMI32
> 
> These last two would provide names to the 'preferred' tables, and
> names that would always exist on all systems, which
> I think is a nice property to have.
> 
> Thoughts?

The idea was discussed before, and discarded. The question is, what use
cases do you envision from a user-space application perspective? As you
found out, the table pointed to by the _SM3_ entry point must be a
super-set of the one pointed to by the _SM_ entry point, so for a
compliant implementation there is no reason to follow the _SM_ entry
point when both exist. And if only one entry point exists, there is
nothing to choose from.

The kernel itself will have to choose one of the entry points when it
comes to dmi_scan and dmi-id. It can't provide two sets of DMI strings.
So it seems reasonable to only expose though sysfs that one entry point
and table that the kernel itself used.

The only case where it would make some sense to expose everything to
user-space is for BIOS debugging purpose: if both entry points exist
and point to different tables, and if the _SM3_ table is broken and
the _SM_ table is correct, then you may want to be able to temporarily
read the _SM_ table instead of the _SM3_ table, to check what needs to
be fixed in the latter. But I would argue that this is beyond the scope
of our code: neither the kernel nor dmidecode are DMI table or BIOS
authoring tools.

If there really ever is a need to ignore the _SM3_ entry point, I'd
rather have a kernel boot parameter for this. This would guarantee that
the kernel and user-space applications always operate on the same data.

So I would keep the sysfs interface simple, as Ivan implemented it.

-- 
Jean Delvare
SUSE L3 Support
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