Xen devel - Jun 2006 - [PATCH 0/2][HVM] Dynamic MP Table Creation

These two patches modify the fully virtualized guests firmware so that
the MP tables are created dynamically based upon the VCPU count.  

Currently the MP tables used by fully virtualized guests are static and
built when the ROMBIOS is built.  If a user wants a different MP table
configuration, HVMLOADER must be rebuilt with new ROMBIOS and MP tables.
These patches are mostly for convenience, especially with binary
distributions of Xen.

The first patch (dynamically-created-mptable.patch) extends HVMLOADER so
that the tables are created when the guest firmware is loaded.  In
addition, the guests are not restricted to 1, 2, 4, and 8-way SMP
configurations.  To do this, a space is allocated inside the ROMBIOS
where the old static MP tables were located and marked with a
''___HVMMP''
signature.  The modified HVMLOADER searches for this signature and
creates the MP tables there.  There is enough space for an MP table with
enough space for 32 processor entries along with the other static
information.  With respect to the MP table, the only thing that is
dynamically created is the number of processor entries.  Everything else
is static and based upon the static MP table information found in the
ROMBIOS code.  Some possible rooms for improvement include more accurate
CPU information that matches what the guest sees via CPUID and an
overall more accurate reflection of the guest''s configuration.

The second patch (hvmloader-mptable-option.patch) adds an hvm builder
option, ''mptable'', that gates the creation of the MP tables
regardless
of the number of VCPUs.  This option can keep MP tables from being
created.  Some of the more modern operating systems can use the ACPI
MADT to determine the SMP configuration and don''t require MP table
support.

The APIC option must be enabled for the MP tables to be created.

I''d like any feedback about these patches and if anyone sees any
problems or objects to this approach.  I know there were previous
problems when a UP Windows guest was trying to be installed/booted with
an MP table.  If there are no objections, please apply to unstable.

Applies cleanly to xen-unstable.hg c/s 10482.

  --travis 



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On 27 Jun 2006, at 07:00, Betak, Travis wrote:
> I''d like any feedback about these patches and if anyone sees any
> problems or objects to this approach.  I know there were previous
> problems when a UP Windows guest was trying to be installed/booted with
> an MP table.  If there are no objections, please apply to unstable.
The first patch looks fine to me, although I''ll wait a day or two and 
see if anyone else has any comments before applying.

The second patch is more doubtful -- can it ever hurt to create the MP 
tables? If any really old OSes get confused by them then you could boot 
them with apic=0. I''d like to avoid adding yet another low-level hacky 
config option unless it is genuinely useful.

  -- Keir


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Keir Fraser wrote:
> On 27 Jun 2006, at 07:00, Betak, Travis wrote:
> 
> > I''d like any feedback about these patches and if anyone sees
any
> > problems or objects to this approach.  I know there were previous
> > problems when a UP Windows guest was trying to be installed/booted
> > with an MP table.  If there are no objections, please apply to
> > unstable. 
> 
> The first patch looks fine to me, although I''ll wait a day or two
and
> see if anyone else has any comments before applying.
Sounds good.
> The second patch is more doubtful -- can it ever hurt to create the 
> MP tables? If any really old OSes get confused by them then you
> could boot them with apic=0. I''d like to avoid adding yet another
> low-level hacky config option unless it is genuinely useful.
I agree.  This is more of a "just in case" option.  I haven''t
seen
anything hurt by MP tables being present as long as they accurately 
reflect the configuration.

  --travis



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