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

[Igor Mammedov <imammedo@redhat.com>]

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.

(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)


> +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.

> +    }
> +}
> +
>  /* 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?

>  }
>  
>  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];