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

[Anthony Liguori]

"Michael S. Tsirkin" <address@hidden> writes:

> 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
>
> Signed-off-by: Michael S. Tsirkin <address@hidden>

As best I can tell, we're papering over an ABI breakage in KVM.

If an old QEMU attempts to do a live migration on a new kernel,
migrating to an QEMU on a different box with an older kernel, it will
fail because there is state that isn't being migrated.

This ought to be fixed in the kernel by making these features
whitelisted by userspace.

Regards,

Anthony Liguori


> ---
>  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