Re: [PATCH] target/i386: always create kvmclock device

[Antoine Damhet <antoine.damhet@blade-group.com>]

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On Thu, Sep 17, 2020 at 01:13:06PM +0200, Vitaly Kuznetsov wrote:
> QEMU's kvmclock device is only created when KVM PV feature bits for
> kvmclock (KVM_FEATURE_CLOCKSOURCE/KVM_FEATURE_CLOCKSOURCE2) are
> exposed to the guest. With 'kvm=off' cpu flag the device is not
> created and we don't call KVM_GET_CLOCK/KVM_SET_CLOCK upon migration.
> It was reported that without these call at least Hyper-V TSC page
> clocksouce (which can be enabled independently) gets broken after
> migration.
> 
> Switch to creating kvmclock QEMU device unconditionally, it seems
> to always make sense to call KVM_GET_CLOCK/KVM_SET_CLOCK on migration.
> Use KVM_CAP_ADJUST_CLOCK check instead of CPUID feature bits.
> 
> Reported-by: Antoine Damhet <antoine.damhet@blade-group.com>
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  hw/i386/kvm/clock.c    | 6 +-----
>  target/i386/kvm.c      | 5 +++++
>  target/i386/kvm_i386.h | 1 +
>  3 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
> index 64283358f91d..526c9ea5172b 100644
> --- a/hw/i386/kvm/clock.c
> +++ b/hw/i386/kvm/clock.c
> @@ -330,11 +330,7 @@ static const TypeInfo kvmclock_info = {
>  /* Note: Must be called after VCPU initialization. */
>  void kvmclock_create(void)
>  {
> -    X86CPU *cpu = X86_CPU(first_cpu);
> -
> -    if (kvm_enabled() &&
> -        cpu->env.features[FEAT_KVM] & ((1ULL << KVM_FEATURE_CLOCKSOURCE) |
> -                                       (1ULL << KVM_FEATURE_CLOCKSOURCE2))) {
> +    if (kvm_enabled() && kvm_has_adjust_clock()) {

Shouldn't the old check used when machine type <= 5.1 in order to avoid
migration incompatibility ?

>          sysbus_create_simple(TYPE_KVM_CLOCK, -1, NULL);
>      }
>  }
> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> index 4a8b3a41c1bc..20b31b65307b 100644
> --- a/target/i386/kvm.c
> +++ b/target/i386/kvm.c
> @@ -143,6 +143,11 @@ bool kvm_has_adjust_clock_stable(void)
>      return (ret == KVM_CLOCK_TSC_STABLE);
>  }
>  
> +bool kvm_has_adjust_clock(void)
> +{
> +    return kvm_check_extension(kvm_state, KVM_CAP_ADJUST_CLOCK);
> +}
> +
>  bool kvm_has_exception_payload(void)
>  {
>      return has_exception_payload;
> diff --git a/target/i386/kvm_i386.h b/target/i386/kvm_i386.h
> index 064b8798a26c..0fce4e51d2d6 100644
> --- a/target/i386/kvm_i386.h
> +++ b/target/i386/kvm_i386.h
> @@ -34,6 +34,7 @@
>  
>  bool kvm_allows_irq0_override(void);
>  bool kvm_has_smm(void);
> +bool kvm_has_adjust_clock(void);
>  bool kvm_has_adjust_clock_stable(void);
>  bool kvm_has_exception_payload(void);
>  void kvm_synchronize_all_tsc(void);
> -- 
> 2.25.4
> 
> 

-- 
Antoine 'xdbob' Damhet

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