Re: [Qemu-devel] [PATCH RFC] acpi: don't build acpi tables for xen hvm guests

[Wei Liu]

On Thu, Oct 27, 2016 at 12:03:42PM +0100, Wei Liu wrote:
> Cc Sander
> 
> On Thu, Oct 27, 2016 at 12:54:02PM +0200, Igor Mammedov wrote:
> > On Wed, 26 Oct 2016 16:22:34 +0100
> > Wei Liu <address@hidden> wrote:
> > 
> > > On Wed, Oct 26, 2016 at 05:09:52PM +0200, Igor Mammedov wrote:
> > > > On Tue, 25 Oct 2016 18:28:04 +0100
> > > > Wei Liu <address@hidden> wrote:
> > > >   
> > > > > Xen's toolstack is in charge of building ACPI tables. Skip acpi table
> > > > > building if running on Xen.
> > > > > 
> > > > > This issue is discovered due to direct kernel boot on Xen doesn't boot
> > > > > anymore, because the new ACPI tables cause the guest to exceed its
> > > > > memory allocation limit.
> > > > > 
> > > > > Reported-by: Sander Eikelenboom <address@hidden>
> > > > > Signed-off-by: Wei Liu <address@hidden>  
> > > > Question is:
> > > > Why does xen guest get ACPI tables from QEMU instead of using
> > > > Xen provided ones.
> > > > Maybe it's firmware issue i.e. firmware side shouldn't load
> > > > ACPI tables from QEMU provided fwcfg file and load Xen provided instead.
> > > >   
> > > 
> > > It hasn't come to the point that the guest is booted. QEMU exits when
> > > trying to populate some pages for the guest, at which point the guest
> > > has not yet been started.  In a sense, Xen guest doesn't get ACPI from
> > > QEMU because it never gets to that point.
> > > 
> > > Direct kernel boot causes fw_cfg to be filled in. pcms->has_acpi_build
> > > defaults to true and acpi_enabled is also true. These make all checks in
> > > acpi_setup pass. QEMU proceeds to build and load ACPI tables (which are
> > > never going to be used by Xen guests), causing the guest to exceeds its
> > > limit.
> > > 
> > > Wei.
> > Would something like this fix issue for you?
> > 
> > diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> > index a54a468..61b6026 100644
> > --- a/hw/i386/pc_piix.c
> > +++ b/hw/i386/pc_piix.c
> > @@ -1094,10 +1094,13 @@ DEFINE_PC_MACHINE(isapc, "isapc", pc_init_isa,
> >  #ifdef CONFIG_XEN
> >  static void xenfv_machine_options(MachineClass *m)
> >  {
> > +    PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
> > +
> >      m->desc = "Xen Fully-virtualized PC";
> >      m->max_cpus = HVM_MAX_VCPUS;
> >      m->default_machine_opts = "accel=xen";
> >      m->hot_add_cpu = pc_hot_add_cpu;
> > +    pcmc->has_acpi_build = false;
> >  }
> >  
> >  DEFINE_PC_MACHINE(xenfv, "xenfv", pc_xen_hvm_init,
> > 
> 
> Yes, it does.
> 
> (I have a similar patch in my queue already)
> 

Oh, the reason I didn't send it out is because Eduardo suggested we
should use a new field instead of setting has_acpi_build outside of
PCMachineClass init function.

I have actually gotten around to investigate this option and what is
needed to be done.

Wei.

> Wei.