Re: [PATCH v3 05/10] KVM: x86: Restrict writeback of VCPU state

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Feb 24, 2010 at 03:17:53PM +0100, Jan Kiszka wrote:
> Do not write nmi_pending, sipi_vector, and mpstate unless we at least go
> through a reset. And TSC as well as KVM wallclocks should only be
> written on full sync, otherwise we risk to drop some time on during
> state read-modify-write.
> 
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
>  kvm.h                 |    2 +-
>  qemu-kvm-x86.c        |    2 +-
>  target-i386/kvm.c     |   32 ++++++++++++++++++++------------
>  target-i386/machine.c |    2 +-
>  4 files changed, 23 insertions(+), 15 deletions(-)
> 
> diff --git a/kvm.h b/kvm.h
> index 3ec5b59..3ee307d 100644
> --- a/kvm.h
> +++ b/kvm.h
> @@ -44,7 +44,7 @@ int kvm_log_stop(target_phys_addr_t phys_addr, ram_addr_t size);
>  int kvm_has_sync_mmu(void);
>  int kvm_has_vcpu_events(void);
>  int kvm_has_robust_singlestep(void);
> -int kvm_put_vcpu_events(CPUState *env);
> +int kvm_put_vcpu_events(CPUState *env, int level);
>  int kvm_get_vcpu_events(CPUState *env);
>  
>  void kvm_cpu_register_phys_memory_client(void);
> diff --git a/qemu-kvm-x86.c b/qemu-kvm-x86.c
> index 4e6ae70..b0f9670 100644
> --- a/qemu-kvm-x86.c
> +++ b/qemu-kvm-x86.c
> @@ -1391,7 +1391,7 @@ void kvm_arch_push_nmi(void *opaque)
>  void kvm_arch_cpu_reset(CPUState *env)
>  {
>      kvm_arch_reset_vcpu(env);
> -    kvm_put_vcpu_events(env);
> +    kvm_put_vcpu_events(env, KVM_PUT_RESET_STATE);
>      kvm_reset_mpstate(env);
>      if (!cpu_is_bsp(env) && !kvm_irqchip_in_kernel()) {
>          env->interrupt_request &= ~CPU_INTERRUPT_HARD;
> diff --git a/target-i386/kvm.c b/target-i386/kvm.c
> index 5f0829b..f1f44d3 100644
> --- a/target-i386/kvm.c
> +++ b/target-i386/kvm.c
> @@ -541,7 +541,7 @@ static void kvm_msr_entry_set(struct kvm_msr_entry *entry,
>      entry->data = value;
>  }
>  
> -static int kvm_put_msrs(CPUState *env)
> +static int kvm_put_msrs(CPUState *env, int level)
>  {
>      struct {
>          struct kvm_msrs info;
> @@ -555,7 +555,6 @@ static int kvm_put_msrs(CPUState *env)
>      kvm_msr_entry_set(&msrs[n++], MSR_IA32_SYSENTER_EIP, env->sysenter_eip);
>      if (kvm_has_msr_star(env))
>  	kvm_msr_entry_set(&msrs[n++], MSR_STAR, env->star);
> -    kvm_msr_entry_set(&msrs[n++], MSR_IA32_TSC, env->tsc);
>      kvm_msr_entry_set(&msrs[n++], MSR_VM_HSAVE_PA, env->vm_hsave);
>  #ifdef TARGET_X86_64
>      /* FIXME if lm capable */
> @@ -564,8 +563,12 @@ static int kvm_put_msrs(CPUState *env)
>      kvm_msr_entry_set(&msrs[n++], MSR_FMASK, env->fmask);
>      kvm_msr_entry_set(&msrs[n++], MSR_LSTAR, env->lstar);
>  #endif
> -    kvm_msr_entry_set(&msrs[n++], MSR_KVM_SYSTEM_TIME,  env->system_time_msr);
> -    kvm_msr_entry_set(&msrs[n++], MSR_KVM_WALL_CLOCK,  env->wall_clock_msr);
> +    if (level == KVM_PUT_FULL_STATE) {
> +        kvm_msr_entry_set(&msrs[n++], MSR_IA32_TSC, env->tsc);
> +        kvm_msr_entry_set(&msrs[n++], MSR_KVM_SYSTEM_TIME,
> +                          env->system_time_msr);
> +        kvm_msr_entry_set(&msrs[n++], MSR_KVM_WALL_CLOCK, env->wall_clock_msr);
> +    }
>  
>      msr_data.info.nmsrs = n;
>  
> @@ -783,7 +786,7 @@ static int kvm_get_mp_state(CPUState *env)
>  }
>  #endif
>  
> -int kvm_put_vcpu_events(CPUState *env)
> +int kvm_put_vcpu_events(CPUState *env, int level)
>  {
>  #ifdef KVM_CAP_VCPU_EVENTS
>      struct kvm_vcpu_events events;
> @@ -807,8 +810,11 @@ int kvm_put_vcpu_events(CPUState *env)
>  
>      events.sipi_vector = env->sipi_vector;
>  
> -    events.flags =
> -        KVM_VCPUEVENT_VALID_NMI_PENDING | KVM_VCPUEVENT_VALID_SIPI_VECTOR;
> +    events.flags = 0;
> +    if (level >= KVM_PUT_RESET_STATE) {
> +        events.flags |=
> +            KVM_VCPUEVENT_VALID_NMI_PENDING | KVM_VCPUEVENT_VALID_SIPI_VECTOR;
> +    }
>  
>      return kvm_vcpu_ioctl(env, KVM_SET_VCPU_EVENTS, &events);

What is the reason for write-back of any vcpu-event state for RUNTIME 
case again?

The debug workaround?