Re: [PATCH v3 18/19] i386: provide simple 'hv-default=on' option

[Vitaly Kuznetsov]

Igor Mammedov <imammedo@redhat.com> writes:

> On Thu,  7 Jan 2021 16:14:49 +0100
> Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
>> Enabling Hyper-V emulation for a Windows VM is a tiring experience as it
>> requires listing all currently supported enlightenments ("hv-*" CPU
>> features) explicitly. We do have 'hv-passthrough' mode enabling
>> everything but it can't be used in production as it prevents migration.
>> 
>> Introduce a simple 'hv-default=on' CPU flag enabling all currently supported
>> Hyper-V enlightenments. Later, when new enlightenments get implemented,
>> compat_props mechanism will be used to disable them for legacy machine types,
>> this will keep 'hv-default=on' configurations migratable.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>>  docs/hyperv.txt   | 16 +++++++++++++---
>>  target/i386/cpu.c | 38 ++++++++++++++++++++++++++++++++++++++
>>  target/i386/cpu.h |  5 +++++
>>  3 files changed, 56 insertions(+), 3 deletions(-)
>> 
>> diff --git a/docs/hyperv.txt b/docs/hyperv.txt
>> index 5df00da54fc4..a54c066cab09 100644
>> --- a/docs/hyperv.txt
>> +++ b/docs/hyperv.txt
>> @@ -17,10 +17,20 @@ compatible hypervisor and use Hyper-V specific features.
>>  
>>  2. Setup
>>  =========
>> -No Hyper-V enlightenments are enabled by default by either KVM or QEMU. In
>> -QEMU, individual enlightenments can be enabled through CPU flags, e.g:
>> +All currently supported Hyper-V enlightenments can be enabled by specifying
>> +'hv-default=on' CPU flag:
>>  
>> -  qemu-system-x86_64 --enable-kvm --cpu host,hv_relaxed,hv_vpindex,hv_time, 
>> ...
>> +  qemu-system-x86_64 --enable-kvm --cpu host,hv-default ...
>> +
>> +Alternatively, it is possible to do fine-grained enablement through CPU 
>> flags,
>> +e.g:
>> +
>> +  qemu-system-x86_64 --enable-kvm --cpu host,hv-relaxed,hv-vpindex,hv-time 
>> ...
>
> I'd put here not '...' but rather recommended list of flags, and update
> it every time when new feature added if necessary.
>

This is an example of fine-grained enablement, there is no point to put
all the existing flags there (hv-default is the only recommended way
now, the rest is 'expert'/'debugging').

> (not to mention that if we had it to begin with, then new 'hv-default' won't
> be necessary, I still see it as functionality duplication but I will not 
> oppose it)
>

Unfortunately, upper layer tools don't read this doc and update
themselves to enable new features when they appear. Similarly, if when
these tools use '-machine q35' they get all the new features we add
automatically, right?

>
>> +It is also possible to disable individual enlightenments from the default 
>> list,
>> +this can be used for debugging purposes:
>> +
>> +  qemu-system-x86_64 --enable-kvm --cpu host,hv-default=on,hv-evmcs=off ...
>>  
>>  Sometimes there are dependencies between enlightenments, QEMU is supposed to
>>  check that the supplied configuration is sane.
>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
>> index 48007a876e32..99338de00f78 100644
>> --- a/target/i386/cpu.c
>> +++ b/target/i386/cpu.c
>> @@ -4552,6 +4552,24 @@ static void x86_cpuid_set_tsc_freq(Object *obj, 
>> Visitor *v, const char *name,
>>      cpu->env.tsc_khz = cpu->env.user_tsc_khz = value / 1000;
>>  }
>>  
>> +static bool x86_hv_default_get(Object *obj, Error **errp)
>> +{
>> +    X86CPU *cpu = X86_CPU(obj);
>> +
>> +    return cpu->hyperv_default;
>> +}
>> +
>> +static void x86_hv_default_set(Object *obj, bool value, Error **errp)
>> +{
>> +    X86CPU *cpu = X86_CPU(obj);
>> +
>> +    cpu->hyperv_default = value;
>> +
>> +    if (value) {
>> +        cpu->hyperv_features |= cpu->hyperv_default_features;
>
> s/|="/=/ please,
> i.e. no option overrides whatever was specified before to keep semantics 
> consistent.
>

Hm,

this doesn't matter for the most recent machine type as
hyperv_default_features has all the features but imagine you're running
an older machine type which doesn't have 'hv_feature'. Now your
suggestion is 

if I do:

'hv_default,hv_feature=on' I will get "hyperv_default_features | hv_feature"

but if I do

'hv_feature=on,hv_default' I will just get 'hyperv_default_features'
(as hv_default enablement will overwrite everything)

How is this consistent?

>> +    }
>> +}
>> +
>>  /* Generic getter for "feature-words" and "filtered-features" properties */
>>  static void x86_cpu_get_feature_words(Object *obj, Visitor *v,
>>                                        const char *name, void *opaque,
>> @@ -6955,10 +6973,26 @@ static void x86_cpu_initfn(Object *obj)
>>      object_property_add_alias(obj, "pause_filter", obj, "pause-filter");
>>      object_property_add_alias(obj, "sse4_1", obj, "sse4.1");
>>      object_property_add_alias(obj, "sse4_2", obj, "sse4.2");
>> +    object_property_add_alias(obj, "hv_default", obj, "hv-default");
>>  
>>      if (xcc->model) {
>>          x86_cpu_load_model(cpu, xcc->model);
>>      }
>> +
>> +    /* Hyper-V features enabled with 'hv-default=on' */
>> +    cpu->hyperv_default_features = BIT(HYPERV_FEAT_RELAXED) |
>> +        BIT(HYPERV_FEAT_VAPIC) | BIT(HYPERV_FEAT_TIME) |
>> +        BIT(HYPERV_FEAT_CRASH) | BIT(HYPERV_FEAT_RESET) |
>> +        BIT(HYPERV_FEAT_VPINDEX) | BIT(HYPERV_FEAT_RUNTIME) |
>> +        BIT(HYPERV_FEAT_SYNIC) | BIT(HYPERV_FEAT_STIMER) |
>> +        BIT(HYPERV_FEAT_FREQUENCIES) | BIT(HYPERV_FEAT_REENLIGHTENMENT) |
>> +        BIT(HYPERV_FEAT_TLBFLUSH) | BIT(HYPERV_FEAT_IPI) |
>> +        BIT(HYPERV_FEAT_STIMER_DIRECT);
>> +
>> +    /* Enlightened VMCS is only available on Intel/VMX */
>> +    if (kvm_hv_evmcs_available()) {
>> +        cpu->hyperv_default_features |= BIT(HYPERV_FEAT_EVMCS);
>> +    }
> what if VVM is migrated to another host without evmcs,
> will it change CPUID?
>

Evmcs is tightly coupled with VMX, we can't migrate when it's not
there.

>>  }
>>  
>>  static int64_t x86_cpu_get_arch_id(CPUState *cs)
>> @@ -7285,6 +7319,10 @@ static void x86_cpu_common_class_init(ObjectClass 
>> *oc, void *data)
>>                                x86_cpu_get_crash_info_qom, NULL, NULL, NULL);
>>  #endif
>>  
>> +    object_class_property_add_bool(oc, "hv-default",
>> +                              x86_hv_default_get,
>> +                              x86_hv_default_set);
>> +
>>      for (w = 0; w < FEATURE_WORDS; w++) {
>>          int bitnr;
>>          for (bitnr = 0; bitnr < 64; bitnr++) {
>> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
>> index 6220cb2cabb9..8a484becb6b9 100644
>> --- a/target/i386/cpu.h
>> +++ b/target/i386/cpu.h
>> @@ -1657,6 +1657,11 @@ struct X86CPU {
>>      bool hyperv_synic_kvm_only;
>>      uint64_t hyperv_features;
>>      bool hyperv_passthrough;
>> +
>> +    /* 'hv-default' enablement */
>> +    uint64_t hyperv_default_features;
>> +    bool hyperv_default;
>> +
>>      OnOffAuto hyperv_no_nonarch_cs;
>>      uint32_t hyperv_vendor_id[3];
>>      uint32_t hyperv_interface_id[4];
>

-- 
Vitaly