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

[Wei Liu <wei.liu2@citrix.com>]

On Wed, Oct 26, 2016 at 05:09:52PM +0200, Igor Mammedov wrote:
> On Tue, 25 Oct 2016 18:28:04 +0100
> Wei Liu <wei.liu2@citrix.com> 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 <linux@eikelenboom.it>
> > Signed-off-by: Wei Liu <wei.liu2@citrix.com>
> 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.

> > ---
> > Cc: Anthony PERARD <anthony.perard@citrix.com>
> > Cc: Stefano Stabellini <sstabellini@kernel.org>
> > 
> > RFC because I'm not sure this is the best way to fix it.
> > ---
> >  hw/i386/acpi-build.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> > index a26a4bb..2cdff12 100644
> > --- a/hw/i386/acpi-build.c
> > +++ b/hw/i386/acpi-build.c
> > @@ -45,6 +45,7 @@
> >  #include "sysemu/tpm_backend.h"
> >  #include "hw/timer/mc146818rtc_regs.h"
> >  #include "sysemu/numa.h"
> > +#include "hw/xen/xen.h"
> >  
> >  /* Supported chipsets: */
> >  #include "hw/acpi/piix4.h"
> > @@ -2865,6 +2866,11 @@ void acpi_setup(void)
> >          return;
> >      }
> >  
> > +    if (xen_enabled()) {
> > +        ACPI_BUILD_DPRINTF("Xen enabled. Bailing out.\n");
> > +        return;
> > +    }
> > +
> >      build_state = g_malloc0(sizeof *build_state);
> >  
> >      acpi_set_pci_info();
>