Oliver,
Do you have any numbers for the improvement in guests CLOCK_REALTIME
accuracy across migration, when this is in place?
On Thu, Sep 16, 2021 at 06:15:35PM +0000, Oliver Upton wrote:
> Handling the migration of TSCs correctly is difficult, in part because
> Linux does not provide userspace with the ability to retrieve a (TSC,
> realtime) clock pair for a single instant in time. In lieu of a more
> convenient facility, KVM can report similar information in the kvm_clock
> structure.
>
> Provide userspace with a host TSC & realtime pair iff the realtime clock
> is based on the TSC. If userspace provides KVM_SET_CLOCK with a valid
> realtime value, advance the KVM clock by the amount of elapsed time. Do
> not step the KVM clock backwards, though, as it is a monotonic
> oscillator.
>
> Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Oliver Upton <oupton@xxxxxxxxxx>
> ---
> Documentation/virt/kvm/api.rst | 42 ++++++++++++++++++++++++++-------
> arch/x86/include/asm/kvm_host.h | 3 +++
> arch/x86/kvm/x86.c | 36 +++++++++++++++++++++-------
> include/uapi/linux/kvm.h | 7 +++++-
> 4 files changed, 70 insertions(+), 18 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index a6729c8cf063..d0b9c986cf6c 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -993,20 +993,34 @@ such as migration.
> When KVM_CAP_ADJUST_CLOCK is passed to KVM_CHECK_EXTENSION, it returns the
> set of bits that KVM can return in struct kvm_clock_data's flag member.
>
> -The only flag defined now is KVM_CLOCK_TSC_STABLE. If set, the returned
> -value is the exact kvmclock value seen by all VCPUs at the instant
> -when KVM_GET_CLOCK was called. If clear, the returned value is simply
> -CLOCK_MONOTONIC plus a constant offset; the offset can be modified
> -with KVM_SET_CLOCK. KVM will try to make all VCPUs follow this clock,
> -but the exact value read by each VCPU could differ, because the host
> -TSC is not stable.
> +FLAGS:
> +
> +KVM_CLOCK_TSC_STABLE. If set, the returned value is the exact kvmclock
> +value seen by all VCPUs at the instant when KVM_GET_CLOCK was called.
> +If clear, the returned value is simply CLOCK_MONOTONIC plus a constant
> +offset; the offset can be modified with KVM_SET_CLOCK. KVM will try
> +to make all VCPUs follow this clock, but the exact value read by each
> +VCPU could differ, because the host TSC is not stable.
> +
> +KVM_CLOCK_REALTIME. If set, the `realtime` field in the kvm_clock_data
> +structure is populated with the value of the host's real time
> +clocksource at the instant when KVM_GET_CLOCK was called. If clear,
> +the `realtime` field does not contain a value.
> +
> +KVM_CLOCK_HOST_TSC. If set, the `host_tsc` field in the kvm_clock_data
> +structure is populated with the value of the host's timestamp counter (TSC)
> +at the instant when KVM_GET_CLOCK was called. If clear, the `host_tsc` field
> +does not contain a value.
>
> ::
>
> struct kvm_clock_data {
> __u64 clock; /* kvmclock current value */
> __u32 flags;
> - __u32 pad[9];
> + __u32 pad0;
> + __u64 realtime;
> + __u64 host_tsc;
> + __u32 pad[4];
> };
>
>
> @@ -1023,12 +1037,22 @@ Sets the current timestamp of kvmclock to the value specified in its parameter.
> In conjunction with KVM_GET_CLOCK, it is used to ensure monotonicity on scenarios
> such as migration.
>
> +FLAGS:
> +
> +KVM_CLOCK_REALTIME. If set, KVM will compare the value of the `realtime` field
> +with the value of the host's real time clocksource at the instant when
> +KVM_SET_CLOCK was called. The difference in elapsed time is added to the final
> +kvmclock value that will be provided to guests.
> +
> ::
>
> struct kvm_clock_data {
> __u64 clock; /* kvmclock current value */
> __u32 flags;
> - __u32 pad[9];
> + __u32 pad0;
> + __u64 realtime;
> + __u64 host_tsc;
> + __u32 pad[4];
> };
>
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index be6805fc0260..9c34b5b63e39 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1936,4 +1936,7 @@ int kvm_cpu_dirty_log_size(void);
>
> int alloc_all_memslots_rmaps(struct kvm *kvm);
>
> +#define KVM_CLOCK_VALID_FLAGS \
> + (KVM_CLOCK_TSC_STABLE | KVM_CLOCK_REALTIME | KVM_CLOCK_HOST_TSC)
> +
> #endif /* _ASM_X86_KVM_HOST_H */
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 523c4e5c109f..cb5d5cad5124 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2815,10 +2815,20 @@ static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
> get_cpu();
>
> if (__this_cpu_read(cpu_tsc_khz)) {
> +#ifdef CONFIG_X86_64
> + struct timespec64 ts;
> +
> + if (kvm_get_walltime_and_clockread(&ts, &data->host_tsc)) {
> + data->realtime = ts.tv_nsec + NSEC_PER_SEC * ts.tv_sec;
> + data->flags |= KVM_CLOCK_REALTIME | KVM_CLOCK_HOST_TSC;
> + } else
> +#endif
> + data->host_tsc = rdtsc();
> +
> kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
> &hv_clock.tsc_shift,
> &hv_clock.tsc_to_system_mul);
> - data->clock = __pvclock_read_cycles(&hv_clock, rdtsc());
> + data->clock = __pvclock_read_cycles(&hv_clock, data->host_tsc);
> } else {
> data->clock = get_kvmclock_base_ns() + ka->kvmclock_offset;
> }
> @@ -4062,7 +4072,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> r = KVM_SYNC_X86_VALID_FIELDS;
> break;
> case KVM_CAP_ADJUST_CLOCK:
> - r = KVM_CLOCK_TSC_STABLE;
> + r = KVM_CLOCK_VALID_FLAGS;
> break;
> case KVM_CAP_X86_DISABLE_EXITS:
> r |= KVM_X86_DISABLE_EXITS_HLT | KVM_X86_DISABLE_EXITS_PAUSE |
> @@ -5859,12 +5869,12 @@ static int kvm_vm_ioctl_set_clock(struct kvm *kvm, void __user *argp)
> {
> struct kvm_arch *ka = &kvm->arch;
> struct kvm_clock_data data;
> - u64 now_ns;
> + u64 now_raw_ns;
>
> if (copy_from_user(&data, argp, sizeof(data)))
> return -EFAULT;
>
> - if (data.flags)
> + if (data.flags & ~KVM_CLOCK_REALTIME)
> return -EINVAL;
>
> kvm_hv_invalidate_tsc_page(kvm);
> @@ -5878,11 +5888,21 @@ static int kvm_vm_ioctl_set_clock(struct kvm *kvm, void __user *argp)
> * is slightly ahead) here we risk going negative on unsigned
> * 'system_time' when 'data.clock' is very small.
> */
> - if (kvm->arch.use_master_clock)
> - now_ns = ka->master_kernel_ns;
> + if (data.flags & KVM_CLOCK_REALTIME) {
> + u64 now_real_ns = ktime_get_real_ns();
> +
> + /*
> + * Avoid stepping the kvmclock backwards.
> + */
> + if (now_real_ns > data.realtime)
> + data.clock += now_real_ns - data.realtime;
> + }
Forward jumps can also cause problems, for example:
* Kernel watchdogs
* https://patchwork.ozlabs.org/project/qemu-devel/patch/20130618233825.GA19042@xxxxxxxx/
So perhaps limiting the amount of forward jump that is allowed
would be a good thing? (which can happen if the two hosts realtime
clocks are off).
Now by how much, i am not sure.
Or, as mentioned earlier, only enable KVM_CLOCK_REALTIME if userspace
KVM code checks clock synchronization.
Thomas, CC'ed, has deeper understanding of problems with
forward time jumps than I do. Thomas, any comments?
As a note: this makes it not OK to use KVM_CLOCK_REALTIME flag
for either vm pause / vm resume (well, if paused for long periods of time)
or savevm / restorevm.
> + if (ka->use_master_clock)
> + now_raw_ns = ka->master_kernel_ns;
> else
> - now_ns = get_kvmclock_base_ns();
> - ka->kvmclock_offset = data.clock - now_ns;
> + now_raw_ns = get_kvmclock_base_ns();
> + ka->kvmclock_offset = data.clock - now_raw_ns;
> kvm_end_pvclock_update(kvm);
> return 0;
> }
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index a067410ebea5..d228bf394465 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1223,11 +1223,16 @@ struct kvm_irqfd {
>
> /* Do not use 1, KVM_CHECK_EXTENSION returned it before we had flags. */
> #define KVM_CLOCK_TSC_STABLE 2
> +#define KVM_CLOCK_REALTIME (1 << 2)
> +#define KVM_CLOCK_HOST_TSC (1 << 3)
>
> struct kvm_clock_data {
> __u64 clock;
> __u32 flags;
> - __u32 pad[9];
> + __u32 pad0;
> + __u64 realtime;
> + __u64 host_tsc;
> + __u32 pad[4];
> };
>
> /* For KVM_CAP_SW_TLB */
> --
> 2.33.0.309.g3052b89438-goog
>
>
