Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

[Marcelo Tosatti]

On Thu, Dec 03, 2020 at 07:11:16PM +0200, Maxim Levitsky wrote:
> These two new ioctls allow to more precisly capture and
> restore guest's TSC state.
> 
> Both ioctls are meant to be used to accurately migrate guest TSC
> even when there is a significant downtime during the migration.
> 
> Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> ---
>  Documentation/virt/kvm/api.rst | 65 ++++++++++++++++++++++++++++++
>  arch/x86/kvm/x86.c             | 73 ++++++++++++++++++++++++++++++++++
>  include/uapi/linux/kvm.h       | 15 +++++++
>  3 files changed, 153 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 70254eaa5229f..ebecfe4b414ce 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -4826,6 +4826,71 @@ If a vCPU is in running state while this ioctl is invoked, the vCPU may
>  experience inconsistent filtering behavior on MSR accesses.
>  
>  
> +4.127 KVM_GET_TSC_STATE
> +----------------------------
> +
> +:Capability: KVM_CAP_PRECISE_TSC
> +:Architectures: x86
> +:Type: vcpu ioctl
> +:Parameters: struct kvm_tsc_state
> +:Returns: 0 on success, < 0 on error
> +
> +::
> +
> +  #define KVM_TSC_STATE_TIMESTAMP_VALID 1
> +  #define KVM_TSC_STATE_TSC_ADJUST_VALID 2
> +  struct kvm_tsc_state {
> +	__u32 flags;
> +	__u64 nsec;
> +	__u64 tsc;
> +	__u64 tsc_adjust;
> +  };
> +
> +flags values for ``struct kvm_tsc_state``:
> +
> +``KVM_TSC_STATE_TIMESTAMP_VALID``
> +
> +  ``nsec`` contains nanoseconds from unix epoch.
> +    Always set by KVM_GET_TSC_STATE, might be omitted in KVM_SET_TSC_STATE
> +
> +``KVM_TSC_STATE_TSC_ADJUST_VALID``
> +
> +  ``tsc_adjust`` contains valid IA32_TSC_ADJUST value
> +
> +
> +This ioctl allows the user space to read the guest's IA32_TSC,IA32_TSC_ADJUST,
> +and the current value of host's CLOCK_REALTIME clock in nanoseconds since unix
> +epoch.

Why is CLOCK_REALTIME necessary at all? kvmclock uses the host clock as
a time base, but for TSC it should not be necessary.

> +
> +
> +4.128 KVM_SET_TSC_STATE
> +----------------------------
> +
> +:Capability: KVM_CAP_PRECISE_TSC
> +:Architectures: x86
> +:Type: vcpu ioctl
> +:Parameters: struct kvm_tsc_state
> +:Returns: 0 on success, < 0 on error
> +
> +::
> +
> +This ioctl allows to reconstruct the guest's IA32_TSC and TSC_ADJUST value
> +from the state obtained in the past by KVM_GET_TSC_STATE on the same vCPU.
> +
> +If 'KVM_TSC_STATE_TIMESTAMP_VALID' is set in flags,
> +KVM will adjust the guest TSC value by the time that passed since the moment
> +CLOCK_REALTIME timestamp was saved in the struct and current value of
> +CLOCK_REALTIME, and set the guest's TSC to the new value.

This introduces the wraparound bug in Linux timekeeping, doesnt it?

> +
> +Otherwise KVM will set the guest TSC value to the exact value as given
> +in the struct.
> +
> +if KVM_TSC_STATE_TSC_ADJUST_VALID is set, and guest supports IA32_MSR_TSC_ADJUST,
> +then its value will be set to the given value from the struct.
> +
> +It is assumed that either both ioctls will be run on the same machine,
> +or that source and destination machines have synchronized clocks.



>  5. The kvm_run structure
>  ========================
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a3fdc16cfd6f3..9b8a2fe3a2398 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2438,6 +2438,21 @@ static bool kvm_get_walltime_and_clockread(struct timespec64 *ts,
>  
>  	return gtod_is_based_on_tsc(do_realtime(ts, tsc_timestamp));
>  }
> +
> +
> +static void kvm_get_walltime(u64 *walltime_ns, u64 *host_tsc)
> +{
> +	struct timespec64 ts;
> +
> +	if (kvm_get_walltime_and_clockread(&ts, host_tsc)) {
> +		*walltime_ns = timespec64_to_ns(&ts);
> +		return;
> +	}
> +
> +	*host_tsc = rdtsc();
> +	*walltime_ns = ktime_get_real_ns();
> +}
> +
>  #endif
>  
>  /*
> @@ -3757,6 +3772,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_X86_USER_SPACE_MSR:
>  	case KVM_CAP_X86_MSR_FILTER:
>  	case KVM_CAP_ENFORCE_PV_FEATURE_CPUID:
> +#ifdef CONFIG_X86_64
> +	case KVM_CAP_PRECISE_TSC:
> +#endif
>  		r = 1;
>  		break;
>  	case KVM_CAP_SYNC_REGS:
> @@ -4999,6 +5017,61 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>  	case KVM_GET_SUPPORTED_HV_CPUID:
>  		r = kvm_ioctl_get_supported_hv_cpuid(vcpu, argp);
>  		break;
> +#ifdef CONFIG_X86_64
> +	case KVM_GET_TSC_STATE: {
> +		struct kvm_tsc_state __user *user_tsc_state = argp;
> +		u64 host_tsc;
> +
> +		struct kvm_tsc_state tsc_state = {
> +			.flags = KVM_TSC_STATE_TIMESTAMP_VALID
> +		};
> +
> +		kvm_get_walltime(&tsc_state.nsec, &host_tsc);
> +		tsc_state.tsc = kvm_read_l1_tsc(vcpu, host_tsc);
> +
> +		if (guest_cpuid_has(vcpu, X86_FEATURE_TSC_ADJUST)) {
> +			tsc_state.tsc_adjust = vcpu->arch.ia32_tsc_adjust_msr;
> +			tsc_state.flags |= KVM_TSC_STATE_TSC_ADJUST_VALID;
> +		}
> +
> +		r = -EFAULT;
> +		if (copy_to_user(user_tsc_state, &tsc_state, sizeof(tsc_state)))
> +			goto out;
> +		r = 0;
> +		break;
> +	}
> +	case KVM_SET_TSC_STATE: {
> +		struct kvm_tsc_state __user *user_tsc_state = argp;
> +		struct kvm_tsc_state tsc_state;
> +		u64 host_tsc, wall_nsec;
> +
> +		u64 new_guest_tsc, new_guest_tsc_offset;
> +
> +		r = -EFAULT;
> +		if (copy_from_user(&tsc_state, user_tsc_state, sizeof(tsc_state)))
> +			goto out;
> +
> +		kvm_get_walltime(&wall_nsec, &host_tsc);
> +		new_guest_tsc = tsc_state.tsc;
> +
> +		if (tsc_state.flags & KVM_TSC_STATE_TIMESTAMP_VALID) {
> +			s64 diff = wall_nsec - tsc_state.nsec;
> +			if (diff >= 0)
> +				new_guest_tsc += nsec_to_cycles(vcpu, diff);
> +			else
> +				new_guest_tsc -= nsec_to_cycles(vcpu, -diff);
> +		}
> +
> +		new_guest_tsc_offset = new_guest_tsc - kvm_scale_tsc(vcpu, host_tsc);
> +		kvm_vcpu_write_tsc_offset(vcpu, new_guest_tsc_offset);
> +
> +		if (tsc_state.flags & KVM_TSC_STATE_TSC_ADJUST_VALID)
> +			if (guest_cpuid_has(vcpu, X86_FEATURE_TSC_ADJUST))
> +				vcpu->arch.ia32_tsc_adjust_msr = tsc_state.tsc_adjust;
> +		r = 0;
> +		break;
> +	}
> +#endif
>  	default:
>  		r = -EINVAL;
>  	}
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 886802b8ffba3..bf4c38fd58291 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1056,6 +1056,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_ENFORCE_PV_FEATURE_CPUID 190
>  #define KVM_CAP_SYS_HYPERV_CPUID 191
>  #define KVM_CAP_DIRTY_LOG_RING 192
> +#define KVM_CAP_PRECISE_TSC 193
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
> @@ -1169,6 +1170,16 @@ struct kvm_clock_data {
>  	__u32 pad[9];
>  };
>  
> +
> +#define KVM_TSC_STATE_TIMESTAMP_VALID 1
> +#define KVM_TSC_STATE_TSC_ADJUST_VALID 2
> +struct kvm_tsc_state {
> +	__u32 flags;
> +	__u64 nsec;
> +	__u64 tsc;
> +	__u64 tsc_adjust;
> +};
> +
>  /* For KVM_CAP_SW_TLB */
>  
>  #define KVM_MMU_FSL_BOOKE_NOHV		0
> @@ -1563,6 +1574,10 @@ struct kvm_pv_cmd {
>  /* Available with KVM_CAP_DIRTY_LOG_RING */
>  #define KVM_RESET_DIRTY_RINGS		_IO(KVMIO, 0xc7)
>  
> +/* Available with KVM_CAP_PRECISE_TSC*/
> +#define KVM_SET_TSC_STATE          _IOW(KVMIO,  0xc8, struct kvm_tsc_state)
> +#define KVM_GET_TSC_STATE          _IOR(KVMIO,  0xc9, struct kvm_tsc_state)
> +
>  /* Secure Encrypted Virtualization command */
>  enum sev_cmd_id {
>  	/* Guest initialization commands */
> -- 
> 2.26.2