Re: [Qemu-devel] [PATCH hack dontapply v2 0/7] Dynamic _CST generation

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 25, 2018 at 02:32:11PM +0200, Igor Mammedov wrote:
> On Tue, 10 Jul 2018 03:01:30 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > Now that basic support for guest CPU PM is upstream, I started looking
> > for making it migrateable.  Since a VM can be migrated between different
> > hosts, PM info needs to change each time with move the VM to a different
> > host.
> Considering notification is async, so there will be a window when
> guest will be using old Cstates on new host. What will happen if
> machine is migrated to host that doesn't support a Cstate
> that was used on source when guest was started?

My testing shows mwait with a wrong hint works, presumably it just uses
a lot of power.

> > This adds infrastructure - based on Load/Unload - to support exactly
> > that: QEMU generates AML (changing it on migration) and stores it in
> > reserved memory, OSPM loads _CST from there on demand.
> Cool and very tempting idea but I have 2 worries about this approach:
> 1. How well does it work with Windows based guests?
>    (I've tried something similar but generating new AML from AML itself
>     to get rid of our long if/else chains there to make up Notify target name.
>     I ditched it (unfortunately I don't recall why) )

After trying it, I can tell you why - it's a horrid mess of
unreadable code, with arbitrary restraints on package
length etc.

> 2. (probably biggest issue) Loading dynamically generated AML
>    basically would make all AML code ABI, so that static AML
>    in RAM of old QEMU version would match dynamic generated
>    one on target side with new QEMU (/me generalizing approach beyond _CST support).
>    I'd try to keep our AML version less as much as possible
>    and go this route only if there is no other way to do it.

That's a good point, thanks for bringing this up!

So it seems that we will need to define the ABI, yes. All AML code is
an over-statement though, there are specific entry points
we must maintain, right?

And that in the end isn't fundamentally different from the ABIs
mandated by the ACPI spec.

So I have these ideas to try to mitigate the issues:
- document the ABI (like we have in the ACPI spec)
- use a specific prefix for all external calls (like _ for ACPI spec ones)
- use a specific (different) prefix for all internal loaded calls (like
  [A-Z] for non-ACPI spec ones)

Thoughts?

> > This is a partially functional prototype.
> > 
> > What works:
> >     loading _CST dynamically and reporting it to OSPM
> > 
> > What doesn't:
> >     detecting host configuration and generating correct _CST package
> >     notifying guest about the change to trigger _CST re-evaluation
> >     disabling mwait/halt exists as appropriate
> > 
> > Michael S. Tsirkin (6):
> >   acpi: aml: add aml_register()
> >   acpi: generalize aml_package / aml_varpackage
> >   acpi: aml_load/aml_unload
> >   acpi: export acpi_checksum
> >   acpi: init header without linking
> >   acpi: aml generation for _CST
> >   pc: HACK: acpi: tie in _CST object to Processor
> > 
> >  include/hw/acpi/acpi.h      |   2 +
> >  include/hw/acpi/aml-build.h |  14 ++-
> >  include/hw/acpi/cst.h       |   8 ++
> >  include/hw/i386/pc.h        |   5 ++
> >  hw/acpi/aml-build.c         |  81 ++++++++++++++---
> >  hw/acpi/core.c              |   2 +-
> >  hw/acpi/cpu.c               |   5 ++
> >  hw/acpi/cpu_hotplug.c       |  11 +--
> >  hw/acpi/cst.c               | 173 ++++++++++++++++++++++++++++++++++++
> >  hw/arm/virt-acpi-build.c    |   2 +-
> >  hw/i386/acpi-build.c        |  10 ++-
> >  hw/acpi/Makefile.objs       |   2 +-
> >  12 files changed, 290 insertions(+), 25 deletions(-)
> >  create mode 100644 include/hw/acpi/cst.h
> >  create mode 100644 hw/acpi/cst.c
> >