Re: [Qemu-devel] [PATCH 1/1] mach-virt: Change default cpu and gic-version setting to "max"

[Wei Huang <wei@redhat.com>]

On 04/09/2018 10:56 AM, Daniel P. Berrangé wrote:
> On Mon, Apr 09, 2018 at 10:49:21AM -0500, Wei Huang wrote:
>> Running mach-virt machine types (i.e. "-M virt") on different systems can
>> result in various misleading warnings if -cpu and/or gic-version not specified.
>> For KVM, this can be solved mostly by using "host" type. But the "host" type
>> doesn't work for TCG. Compared with "host", the "max" type not only supports
>> auto detection under KVM mode, but also works with TCG. So this patch set
>> "max" as the default types for both -cpu and gic-version.
> 
> Hmm, generally we aim for the config provided by a machine type to be stable
> across QEMU versions.

I understand this principle. But in reality, under KVM mode, the default
config most time doesn't work. If end users specify cpu type manually,
it still doesn't work because the host CPU is vendor-specific design
(e.g. "cortex-a57" doesn't work on QCOM's machine). So we end up with
using "-cpu host" all the time. My argument for this patch is that "-cpu
max" isn't worse than "-cpu host".

> 
> By specifying "max", the machine type will potentially change if new QEMU
> turns on new features in the "max" CPU model, as well as also varying
> depending on what the host OS supports.
> 
> This is a general conceptual problem with the "host" CPU model for all
> target arches and is unfixable by design. This is fine if you accept
> the caveats with using "-cpu host" and opt-in to using it knowing the
> tradeoffs. I'm just not convinced it is reasonable to make "-cpu host"
> be the default value for a machine type out of the box.
> 
> 
>> Signed-off-by: Wei Huang <wei@redhat.com>
>> ---
>>  hw/arm/virt.c | 10 +++++-----
>>  1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
>> index 94dcb125d3..1a9d68b8d5 100644
>> --- a/hw/arm/virt.c
>> +++ b/hw/arm/virt.c
>> @@ -1555,7 +1555,7 @@ static void virt_machine_class_init(ObjectClass *oc, void *data)
>>      mc->minimum_page_bits = 12;
>>      mc->possible_cpu_arch_ids = virt_possible_cpu_arch_ids;
>>      mc->cpu_index_to_instance_props = virt_cpu_index_to_props;
>> -    mc->default_cpu_type = ARM_CPU_TYPE_NAME("cortex-a15");
>> +    mc->default_cpu_type = ARM_CPU_TYPE_NAME("max");
>>      mc->get_default_cpu_node_id = virt_get_default_cpu_node_id;
>>  }
>>  
>> @@ -1609,13 +1609,13 @@ static void virt_2_12_instance_init(Object *obj)
>>                                      "Set on/off to enable/disable using "
>>                                      "physical address space above 32 bits",
>>                                      NULL);
>> -    /* Default GIC type is v2 */
>> -    vms->gic_version = 2;
>> +    /* Default GIC type is max */
>> +    vms->gic_version = -1;
>>      object_property_add_str(obj, "gic-version", virt_get_gic_version,
>>                          virt_set_gic_version, NULL);
>>      object_property_set_description(obj, "gic-version",
>> -                                    "Set GIC version. "
>> -                                    "Valid values are 2, 3 and host", NULL);
>> +                                    "Set GIC version. Valid values are 2, 3, "
>> +                                    "host, and max", NULL);
>>  
>>      if (vmc->no_its) {
>>          vms->its = false;
>> -- 
>> 2.14.3
>>
>>
> 
> Regards,
> Daniel
>