Re: [Qemu-devel] [PATCH v6 3/3] target-i386: add support to migrate vcpu's TSC rate

[Eduardo Habkost]

On Tue, Nov 24, 2015 at 11:33:57AM +0800, Haozhong Zhang wrote:
> This patch enables migrating vcpu's TSC rate. If KVM on the destination
> machine supports TSC scaling, guest programs will observe a consistent
> TSC rate across the migration.
> 
> If TSC scaling is not supported on the destination machine, the
> migration will not be aborted and QEMU on the destination will not set
> vcpu's TSC rate to the migrated value.
> 
> If vcpu's TSC rate specified by CPU option 'tsc-freq' on the destination
> machine is inconsistent with the migrated TSC rate, the migration will
> be aborted.
> 
> For backwards compatibility, the migration of vcpu's TSC rate is
> disabled on pc-*-2.4 and older machine types.
> 
> Signed-off-by: Haozhong Zhang <address@hidden>

Assuming the PC compat code will be moved to
pc_*_2_5_machine_options(), because the patch will be included
after QEMU 2.5.0:

Reviewed-by: Eduardo Habkost <address@hidden>

One comment below:

> ---
[...]
> diff --git a/target-i386/kvm.c b/target-i386/kvm.c
> index 1e811ee..2a0fd54 100644
> --- a/target-i386/kvm.c
> +++ b/target-i386/kvm.c
> @@ -2381,6 +2381,28 @@ int kvm_arch_put_registers(CPUState *cpu, int level)
>          }
>      }
>  
> +    if (level == KVM_PUT_FULL_STATE) {
> +        /* kvm_arch_set_tsc_khz() below can be called in two control flows 
> and
> +         * we don't need to handle its errors in both of them.
> +         *
> +         * One is the control flow that creates a vcpu, where
> +         * kvm_arch_set_tsc_khz() has already been called once before by
> +         * kvm_arch_init_vcpu(). The latter will abort the control flow if 
> there
> +         * are any errors of kvm_arch_set_tsc_khz(). Thus, in this control 
> flow,
> +         * kvm_arch_set_tsc_khz() below never fails and we can safely ignore 
> its
> +         * return values here.
> +         *
> +         * Another is the control flow of migration that sets vcpu's TSC
> +         * frequency on the destination. The only error that can fail the
> +         * migration is the mismatch between the migrated and the 
> user-specified
> +         * TSC frequencies, which has been handled by cpu_post_load(). Other
> +         * errors, i.e. those from kvm_arch_set_tsc_khz(), never fail the
> +         * migration, so we also safely ignore its return values in this 
> control
> +         * flow.
> +         */

This could be more succint. Something like:

/* We don't check for kvm_arch_set_tsc_khz() errors here, because
 * TSC frequency mismatch shouldn't abort migration, unless the
 * user explicitly asked for a more strict TSC setting (e.g.
 * using an explicit "tsc-freq" option).
 */

No need to resubmit because of that, though. The comment can be
changed when applying the patch.

> +        kvm_arch_set_tsc_khz(cpu);
> +    }
> +
>      ret = kvm_getput_regs(x86_cpu, 1);
>      if (ret < 0) {
>          return ret;
> diff --git a/target-i386/machine.c b/target-i386/machine.c
> index a18e16e..e560ca3 100644
> --- a/target-i386/machine.c
> +++ b/target-i386/machine.c
> @@ -6,6 +6,8 @@
>  #include "cpu.h"
>  #include "sysemu/kvm.h"
>  
> +#include "qemu/error-report.h"
> +
>  static const VMStateDescription vmstate_segment = {
>      .name = "segment",
>      .version_id = 1,
> @@ -331,6 +333,13 @@ static int cpu_post_load(void *opaque, int version_id)
>      CPUX86State *env = &cpu->env;
>      int i;
>  
> +    if (env->tsc_khz && env->user_tsc_khz &&
> +        env->tsc_khz != env->user_tsc_khz) {
> +        error_report("Mismatch between user-specified TSC frequency and "
> +                     "migrated TSC frequency");
> +        return -EINVAL;
> +    }
> +
>      /*
>       * Real mode guest segments register DPL should be zero.
>       * Older KVM version were setting it wrongly.
> @@ -775,6 +784,26 @@ static const VMStateDescription vmstate_xss = {
>      }
>  };
>  
> +static bool tsc_khz_needed(void *opaque)
> +{
> +    X86CPU *cpu = opaque;
> +    CPUX86State *env = &cpu->env;
> +    MachineClass *mc = MACHINE_GET_CLASS(qdev_get_machine());
> +    PCMachineClass *pcmc = PC_MACHINE_CLASS(mc);
> +    return env->tsc_khz && pcmc->save_tsc_khz;
> +}
> +
> +static const VMStateDescription vmstate_tsc_khz = {
> +    .name = "cpu/tsc_khz",
> +    .version_id = 1,
> +    .minimum_version_id = 1,
> +    .needed = tsc_khz_needed,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_INT64(env.tsc_khz, X86CPU),
> +        VMSTATE_END_OF_LIST()
> +    }
> +};
> +
>  VMStateDescription vmstate_x86_cpu = {
>      .name = "cpu",
>      .version_id = 12,
> @@ -895,6 +924,7 @@ VMStateDescription vmstate_x86_cpu = {
>          &vmstate_msr_hyperv_runtime,
>          &vmstate_avx512,
>          &vmstate_xss,
> +        &vmstate_tsc_khz,
>          NULL
>      }
>  };
> -- 
> 2.4.8
> 

-- 
Eduardo