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

From: Oliver Upton
Date: Mon Dec 07 2020 - 12:30:28 EST

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On Thu, Dec 3, 2020 at 11:12 AM Maxim Levitsky <mlevitsk@xxxxxxxxxx> 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.
> +
> +
> +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.
> +
> +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);

How would a VMM maintain the phase relationship between guest TSCs
using these ioctls?

For as bugged as the old way of doing things is (i.e. the magic
handling of IA32_TSC), it was at least possible to keep guest TSCs in
sync across a live migration so long as you satisfied the conditions
where KVM decides to fudge the TSCs on your behalf. However, if we
migrate the TSCs by value with an optional timestamp to account for
elapsed time, it would appear that the guest TSC offset is likely to
be inconsistent across vCPUs as the offset calculations above do not
use a fixed host tsc timestamp across all vCPUs.

The reason I'd suggested yielding control of the tsc offset controls
to userspace is that they're unaffected by such variations in per-vCPU
calculations. Not only that, userspace can decide how it wants TSCs to
be handled across a migration explicitly instead of relying on the
above computation being done in the kernel.

> +
> + 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;

How is this ioctl's handling of the TSC_ADJUST msr an improvement over
KVM_{GET,SET}_MSRS like before? It seems orthogonal to the rest of the
intended API as it isn't involved your computation above.

> + 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
>

Next message: Lukas Bulwahn: "[PATCH] RDMA/restrack: update kernel documentation for ib_create_named_qp()"
Previous message: Shakeel Butt: "Re: [PATCH] mm/page_alloc: simplify kmem cgroup charge/uncharge code"
In reply to: Andy Lutomirski: "Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE"
Next in thread: Maxim Levitsky: "[PATCH v2 2/3] KVM: x86: introduce KVM_X86_QUIRK_TSC_HOST_ACCESS"
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