Re: [Qemu-devel] [PATCHv4 3/4] cpuid: disable pv eoi for 1.1 and older compat types

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

On Tue, Aug 28, 2012 at 04:13:38PM -0300, Eduardo Habkost wrote:
> On Tue, Aug 28, 2012 at 08:43:52PM +0300, Michael S. Tsirkin wrote:
> > In preparation for adding PV EOI support, disable PV EOI by default for
> > 1.1 and older machine types, to avoid CPUID changing during migration.
> > 
> > PV EOI can still be enabled/disabled by specifying it explicitly.
> >     Enable for 1.1
> >     -M pc-1.1 -cpu kvm64,+kvm_pv_eoi
> >     Disable for 1.2
> >     -M pc-1.2 -cpu kvm64,-kvm_pv_eoi
> > 
> 
> What about users that are already running "qemu-1.1 -M pc-1.1" on a host
> kernel that supports PV EOI already? They would get PV EOI disabled when
> migrating to a destination running "qemu-1.2 -M pc-1.1".
> 
> (On the other hand, people running "qemu-1.1 -M pc-1.1" on a host kernel
> supporting PV EOI already have migration broken, so there's not much we
> can do for them)

Exactly.

Talked to Gleb, long term I think we should rework code to make
it forward-compatible wrt adding new MSRs:
- source gets list of MSRs to be migrated from KVM and simply sends them all
- send all MSRS in key/value format
- destination gets list of MSRs to be migrated from KVM and
  only restores the supported ones
Too late for 1.2?

> While we don't make the KVM feature-bit handling sane (with defaults
> that are not blindly derived from the host kernel capabilities), maybe
> the safest bet is to expect users to not migrate between hosts running
> kernels with different KVM capabilities? (I am not sure which option is
> better)

Sorry not sure what you talk about here. What has KVM feature-bit
handling to do with this patchset?

> 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  hw/Makefile.objs  |  2 +-
> >  hw/cpu_flags.c    | 32 ++++++++++++++++++++++++++++++++
> >  hw/cpu_flags.h    |  9 +++++++++
> >  hw/pc_piix.c      |  2 ++
> >  target-i386/cpu.c |  8 ++++++++
> >  5 files changed, 52 insertions(+), 1 deletion(-)
> >  create mode 100644 hw/cpu_flags.c
> >  create mode 100644 hw/cpu_flags.h
> > 
> > diff --git a/hw/Makefile.objs b/hw/Makefile.objs
> > index 850b87b..3f2532a 100644
> > --- a/hw/Makefile.objs
> > +++ b/hw/Makefile.objs
> > @@ -1,5 +1,5 @@
> >  hw-obj-y = usb/ ide/
> > -hw-obj-y += loader.o
> > +hw-obj-y += loader.o cpu_flags.o
> >  hw-obj-$(CONFIG_VIRTIO) += virtio-console.o
> >  hw-obj-$(CONFIG_VIRTIO_PCI) += virtio-pci.o
> >  hw-obj-y += fw_cfg.o
> > diff --git a/hw/cpu_flags.c b/hw/cpu_flags.c
> > new file mode 100644
> > index 0000000..7a633c0
> > --- /dev/null
> > +++ b/hw/cpu_flags.c
> > @@ -0,0 +1,32 @@
> > +/*
> > + * CPU compatibility flags.
> > + *
> > + * Copyright (c) 2012 Red Hat Inc.
> > + * Author: Michael S. Tsirkin.
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License as published by
> > + * the Free Software Foundation; either version 2 of the License, or
> > + * (at your option) any later version.
> > + *
> > + * This program is distributed in the hope that it will be useful,
> > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> > + * GNU General Public License for more details.
> > + *
> > + * You should have received a copy of the GNU General Public License along
> > + * with this program; if not, see <http://www.gnu.org/licenses/>.
> > + */
> > +#include "hw/cpu_flags.h"
> > +
> > +static bool kvm_pv_eoi_disabled_state;
> > +
> > +void disable_kvm_pv_eoi(void)
> > +{
> > +       kvm_pv_eoi_disabled_state = true;
> > +}
> > +
> > +bool kvm_pv_eoi_disabled(void)
> > +{
> > +       return kvm_pv_eoi_disabled_state;
> > +}
> > diff --git a/hw/cpu_flags.h b/hw/cpu_flags.h
> > new file mode 100644
> > index 0000000..05777b6
> > --- /dev/null
> > +++ b/hw/cpu_flags.h
> > @@ -0,0 +1,9 @@
> > +#ifndef HW_CPU_FLAGS_H
> > +#define HW_CPU_FLAGS_H
> > +
> > +#include <stdbool.h>
> > +
> > +void disable_kvm_pv_eoi(void);
> > +bool kvm_pv_eoi_disabled(void);
> > +
> > +#endif
> > diff --git a/hw/pc_piix.c b/hw/pc_piix.c
> > index 008d42f..bdbceda 100644
> > --- a/hw/pc_piix.c
> > +++ b/hw/pc_piix.c
> > @@ -46,6 +46,7 @@
> >  #ifdef CONFIG_XEN
> >  #  include <xen/hvm/hvm_info_table.h>
> >  #endif
> > +#include "cpu_flags.h"
> >  
> >  #define MAX_IDE_BUS 2
> >  
> > @@ -371,6 +372,7 @@ static QEMUMachine pc_machine_v1_2 = {
> >  
> >  static void pc_machine_v1_1_compat(void)
> >  {
> > +    disable_kvm_pv_eoi();
> >  }
> >  
> >  static void pc_init_pci_v1_1(ram_addr_t ram_size,
> > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > index 120a2e3..0d02fd1 100644
> > --- a/target-i386/cpu.c
> > +++ b/target-i386/cpu.c
> > @@ -23,6 +23,7 @@
> >  
> >  #include "cpu.h"
> >  #include "kvm.h"
> > +#include "asm/kvm_para.h"
> >  
> >  #include "qemu-option.h"
> >  #include "qemu-config.h"
> > @@ -33,6 +34,7 @@
> >  #include "hyperv.h"
> >  
> >  #include "hw/hw.h"
> > +#include "hw/cpu_flags.h"
> >  
> >  /* feature flags taken from "Intel Processor Identification and the CPUID
> >   * Instruction" and AMD's "CPUID Specification".  In cases of disagreement
> > @@ -889,6 +891,12 @@ static int cpu_x86_find_by_name(x86_def_t *x86_cpu_def, const char *cpu_model)
> >  
> >      plus_kvm_features = ~0; /* not supported bits will be filtered out later */
> >  
> > +    /* Disable PV EOI for old machine types.
> > +     * Feature flags can still override. */
> > +    if (kvm_pv_eoi_disabled()) {
> > +        plus_kvm_features &= ~(0x1 << KVM_FEATURE_PV_EOI);
> > +    }
> > +
> >      add_flagname_to_bitmaps("hypervisor", &plus_features,
> >          &plus_ext_features, &plus_ext2_features, &plus_ext3_features,
> >          &plus_kvm_features, &plus_svm_features);
> > -- 
> > MST
> > 
> > 
> 
> -- 
> Eduardo