Re: [PATCH 2/2] KVM: Use -cpu best as default on x86

[Ryan Harper <ryanh@us.ibm.com>]

* Alexander Graf <agraf@suse.de> [2012-01-08 17:53]:
> When running QEMU without -cpu parameter, the user usually wants a sane
> default. So far, we're using the qemu64/qemu32 CPU type, which basically
> means "the maximum TCG can emulate".

it also means we all maximum possible migration targets.  Have you
given any thought to migration with -cpu best? 

> 
> That's a really good default when using TCG, but when running with KVM
> we much rather want a default saying "the maximum performance I can get".
> 
> Fortunately we just added an option that gives us the best performance
> while still staying safe on the testability side of things: -cpu best.
> So all we need to do is make -cpu best the default when the user doesn't
> define any.
> 
> This fixes a lot of subtile breakage in the GNU toolchain (libgmp) which
> hicks up on QEMU's non-existent CPU models.
> 
> This patch also adds a new pc-1.1 machine type to keep backwards compatible
> with older versions of QEMU.
> 
> Signed-off-by: Alexander Graf <agraf@suse.de>
> ---
>  hw/pc_piix.c |   42 ++++++++++++++++++++++++++++++++++--------
>  1 files changed, 34 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/pc_piix.c b/hw/pc_piix.c
> index 00f525e..3d78ccb 100644
> --- a/hw/pc_piix.c
> +++ b/hw/pc_piix.c
> @@ -79,7 +79,8 @@ static void pc_init1(MemoryRegion *system_memory,
>                       const char *initrd_filename,
>                       const char *cpu_model,
>                       int pci_enabled,
> -                     int kvmclock_enabled)
> +                     int kvmclock_enabled,
> +                     int may_cpu_best)
>  {
>      int i;
>      ram_addr_t below_4g_mem_size, above_4g_mem_size;
> @@ -102,6 +103,9 @@ static void pc_init1(MemoryRegion *system_memory,
>      MemoryRegion *rom_memory;
>      DeviceState *dev;
> 
> +    if (!cpu_model && kvm_enabled() && may_cpu_best) {
> +        cpu_model = "best";
> +    }
>      pc_cpus_init(cpu_model);
> 
>      if (kvmclock_enabled) {
> @@ -263,7 +267,21 @@ static void pc_init_pci(ram_addr_t ram_size,
>               get_system_io(),
>               ram_size, boot_device,
>               kernel_filename, kernel_cmdline,
> -             initrd_filename, cpu_model, 1, 1);
> +             initrd_filename, cpu_model, 1, 1, 1);
> +}
> +
> +static void pc_init_pci_oldcpu(ram_addr_t ram_size,
> +                               const char *boot_device,
> +                               const char *kernel_filename,
> +                               const char *kernel_cmdline,
> +                               const char *initrd_filename,
> +                               const char *cpu_model)
> +{
> +    pc_init1(get_system_memory(),
> +             get_system_io(),
> +             ram_size, boot_device,
> +             kernel_filename, kernel_cmdline,
> +             initrd_filename, cpu_model, 1, 1, 0);
>  }
> 
>  static void pc_init_pci_no_kvmclock(ram_addr_t ram_size,
> @@ -277,7 +295,7 @@ static void pc_init_pci_no_kvmclock(ram_addr_t ram_size,
>               get_system_io(),
>               ram_size, boot_device,
>               kernel_filename, kernel_cmdline,
> -             initrd_filename, cpu_model, 1, 0);
> +             initrd_filename, cpu_model, 1, 0, 0);
>  }
> 
>  static void pc_init_isa(ram_addr_t ram_size,
> @@ -293,7 +311,7 @@ static void pc_init_isa(ram_addr_t ram_size,
>               get_system_io(),
>               ram_size, boot_device,
>               kernel_filename, kernel_cmdline,
> -             initrd_filename, cpu_model, 0, 1);
> +             initrd_filename, cpu_model, 0, 1, 0);
>  }
> 
>  #ifdef CONFIG_XEN
> @@ -314,8 +332,8 @@ static void pc_xen_hvm_init(ram_addr_t ram_size,
>  }
>  #endif
> 
> -static QEMUMachine pc_machine_v1_0 = {
> -    .name = "pc-1.0",
> +static QEMUMachine pc_machine_v1_1 = {
> +    .name = "pc-1.1",
>      .alias = "pc",
>      .desc = "Standard PC",
>      .init = pc_init_pci,
> @@ -323,17 +341,24 @@ static QEMUMachine pc_machine_v1_0 = {
>      .is_default = 1,
>  };
> 
> +static QEMUMachine pc_machine_v1_0 = {
> +    .name = "pc-1.0",
> +    .desc = "Standard PC",
> +    .init = pc_init_pci_oldcpu,
> +    .max_cpus = 255,
> +};
> +
>  static QEMUMachine pc_machine_v0_15 = {
>      .name = "pc-0.15",
>      .desc = "Standard PC",
> -    .init = pc_init_pci,
> +    .init = pc_init_pci_oldcpu,
>      .max_cpus = 255,
>  };
> 
>  static QEMUMachine pc_machine_v0_14 = {
>      .name = "pc-0.14",
>      .desc = "Standard PC",
> -    .init = pc_init_pci,
> +    .init = pc_init_pci_oldcpu,
>      .max_cpus = 255,
>      .compat_props = (GlobalProperty[]) {
>          {
> @@ -612,6 +637,7 @@ static QEMUMachine xenfv_machine = {
> 
>  static void pc_machine_init(void)
>  {
> +    qemu_register_machine(&pc_machine_v1_1);
>      qemu_register_machine(&pc_machine_v1_0);
>      qemu_register_machine(&pc_machine_v0_15);
>      qemu_register_machine(&pc_machine_v0_14);
> -- 
> 1.6.0.2
> 

-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
ryanh@us.ibm.com