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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 26 Oct 2016, Wei Liu 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 <wei.liu2@xxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> > > Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> > 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.

What if libxl passes -no-acpi to QEMU?


> > > ---
> > > Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> > > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > 
> > > 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();
> > 
> 

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