Re: [PATCH v2] i386: Make migration fail when Hyper-V reenlightenment was enabled but 'user_tsc_khz' is unset

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Vitaly Kuznetsov (vkuznets@redhat.com) wrote:
> KVM doesn't fully support Hyper-V reenlightenment notifications on
> migration. In particular, it doesn't support emulating TSC frequency
> of the source host by trapping all TSC accesses so unless TSC scaling
> is supported on the destination host and KVM_SET_TSC_KHZ succeeds, it
> is unsafe to proceed with migration.
> 
> KVM_SET_TSC_KHZ is called from two sites: kvm_arch_init_vcpu() and
> kvm_arch_put_registers(). The later (intentionally) doesn't propagate
> errors allowing migrations to succeed even when TSC scaling is not
> supported on the destination. This doesn't suit 're-enlightenment'
> use-case as we have to guarantee that TSC frequency stays constant.
> 
> Require 'tsc-frequency=' command line option to be specified for successful
> migration when re-enlightenment was enabled by the guest.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> This patch is a successor of "[PATCH 3/3] i386: Make sure
> kvm_arch_set_tsc_khz() succeeds on migration when 'hv-reenlightenment'
> was exposed" taking a different approach suggested by Paolo.
> ---
>  docs/hyperv.txt                |  5 +++++
>  target/i386/kvm/hyperv-proto.h |  1 +
>  target/i386/machine.c          | 20 ++++++++++++++++++++
>  3 files changed, 26 insertions(+)
> 
> diff --git a/docs/hyperv.txt b/docs/hyperv.txt
> index 5df00da54fc4..e53c581f4586 100644
> --- a/docs/hyperv.txt
> +++ b/docs/hyperv.txt
> @@ -160,6 +160,11 @@ the hypervisor) until it is ready to switch to the new one. This, in conjunction
>  with hv-frequencies, allows Hyper-V on KVM to pass stable clocksource (Reference
>  TSC page) to its own guests.
>  
> +Note, KVM doesn't fully support re-enlightenment notifications and doesn't
> +emulate TSC accesses after migration so 'tsc-frequency=' CPU option also has to
> +be specified to make migration succeed. The destination host has to either have
> +the same TSC frequency or support TSC scaling CPU feature.
> +
>  Recommended: hv-frequencies
>  
>  3.16. hv-evmcs
> diff --git a/target/i386/kvm/hyperv-proto.h b/target/i386/kvm/hyperv-proto.h
> index 056a305be38c..e30d64b4ade4 100644
> --- a/target/i386/kvm/hyperv-proto.h
> +++ b/target/i386/kvm/hyperv-proto.h
> @@ -139,6 +139,7 @@
>   * Reenlightenment notification MSRs
>   */
>  #define HV_X64_MSR_REENLIGHTENMENT_CONTROL      0x40000106
> +#define HV_REENLIGHTENMENT_ENABLE_BIT           (1u << 16)
>  #define HV_X64_MSR_TSC_EMULATION_CONTROL        0x40000107
>  #define HV_X64_MSR_TSC_EMULATION_STATUS         0x40000108
>  
> diff --git a/target/i386/machine.c b/target/i386/machine.c
> index 7259fe6868c6..137604ddb898 100644
> --- a/target/i386/machine.c
> +++ b/target/i386/machine.c
> @@ -883,11 +883,31 @@ static bool hyperv_reenlightenment_enable_needed(void *opaque)
>          env->msr_hv_tsc_emulation_status != 0;
>  }
>  
> +static int hyperv_reenlightenment_post_load(void *opaque, int version_id)
> +{
> +    X86CPU *cpu = opaque;
> +    CPUX86State *env = &cpu->env;
> +
> +    /*
> +     * KVM doesn't fully support re-enlightenment notifications so we need to
> +     * make sure TSC frequency doesn't change upon migration.
> +     */
> +    if ((env->msr_hv_reenlightenment_control & HV_REENLIGHTENMENT_ENABLE_BIT) &&
> +        !env->user_tsc_khz) {
> +        error_report("Guest enabled re-enlightenment notifications, "
> +                     "'tsc-frequency=' has to be specified");

It's unusual to fail on the destination for a valid configuration but
guest state;  wouldn't it be better to always insist on tsc-frequency if
that hv feature is exposed; failing early before reeiving the state?

Dave

> +        return -EINVAL;
> +    }
> +
> +    return 0;
> +}
> +
>  static const VMStateDescription vmstate_msr_hyperv_reenlightenment = {
>      .name = "cpu/msr_hyperv_reenlightenment",
>      .version_id = 1,
>      .minimum_version_id = 1,
>      .needed = hyperv_reenlightenment_enable_needed,
> +    .post_load = hyperv_reenlightenment_post_load,
>      .fields = (VMStateField[]) {
>          VMSTATE_UINT64(env.msr_hv_reenlightenment_control, X86CPU),
>          VMSTATE_UINT64(env.msr_hv_tsc_emulation_control, X86CPU),
> -- 
> 2.30.2
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK