Re: [PATCH v4 20/21] i386: provide simple 'hv-default=on' option

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 10 Feb 2021 17:40:32 +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 | 52 +++++++++++++++++++++++++++++++++++++++++++++++
>  target/i386/cpu.h |  3 +++
>  3 files changed, 68 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 ...
> +
> +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 f8df2caed779..013aa60272d8 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -4736,6 +4736,12 @@ static void x86_hv_passthrough_set(Object *obj, bool value, Error **errp)
>  {
>      X86CPU *cpu = X86_CPU(obj);
>  
> +    if (cpu->hyperv_default) {
> +        error_setg(errp,
> +                   "'hv-default' and 'hv-paththrough' are mutually exclusive");
> +        return;
> +    }
> +
>      cpu->hyperv_passthrough = value;
>  
>      /* hv-passthrough overrides everything with what's supported by the host */
> @@ -4748,6 +4754,33 @@ static void x86_hv_passthrough_set(Object *obj, bool value, Error **errp)
>      return;
>  }
>  
> +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);
> +
> +    if (cpu->hyperv_passthrough) {
> +        error_setg(errp,
> +                   "'hv-default' and 'hv-paththrough' are mutually exclusive");
this check will work only half way, i.e.: hv-paththrough=on,hv-default=on|off
(where off value looks a bit wierd)
but not other way around: hv-default=on,hv-paththrough=on

were you thinking about following error:
  "hv-default can't be used after hv-paththrough were enabled"

or if it symmetric, then putting this check in realizefn() will do the job
as both properties are processed by that time.

> +        return;
> +    }
> +
> +    cpu->hyperv_default = value;
> +
> +    /* hv-default overrides everything with the default set */
> +    if (value) {
> +        cpu->hyperv_features = cpu->hyperv_default_features;
> +        cpu->hyperv_features_on = 0;
> +        cpu->hyperv_features_off = 0;
> +    }
> +}
> +
>  /* 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,
> @@ -7152,6 +7185,21 @@ static void x86_cpu_initfn(Object *obj)
>      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);
> +    }
>  }
>  
>  static int64_t x86_cpu_get_arch_id(CPUState *cs)
> @@ -7486,6 +7534,10 @@ static void x86_cpu_common_class_init(ObjectClass *oc, void *data)
>                                     x86_hv_passthrough_get,
>                                     x86_hv_passthrough_set);
>  
> +    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 b4fbd46f0fc9..59350e70fb51 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -1670,6 +1670,9 @@ struct X86CPU {
>      uint64_t hyperv_features_on;
>      uint64_t hyperv_features_off;
>      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];