Date:   Tue, 8 Dec 2020 14:35:33 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        open list <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Shuah Khan <shuah@...nel.org>,
        Andrew Jones <drjones@...hat.com>,
        Oliver Upton <oupton@...gle.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

On Tue, Dec 08, 2020 at 04:50:53PM +0200, Maxim Levitsky wrote:
> On Mon, 2020-12-07 at 20:29 -0300, Marcelo Tosatti wrote:
> > 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@...hat.com>
> > > Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> > > ---
> > >  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.
> 
> 
> CLOCK_REALTIME is used as an absolute time reference that should match
> on both computers. I could have used CLOCK_TAI instead for example.
> 
> The reference allows to account for time passed between saving and restoring
> the TSC as explained above.

As mentioned we don't want this due to the overflow. 

Again, i think higher priority is to allow enablement of invariant TSC
by default (to disable kvmclock).

> > > +
> > > +
> > > +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?
> 
> It does.
> Could you prepare a reproducer for this bug so I get a better idea about
> what are you talking about?

Enable CONFIG_DEBUG_TIMEKEEPING, check what max_cycles is from the TSC
clocksource:

#ifdef CONFIG_DEBUG_TIMEKEEPING
#define WARNING_FREQ (HZ*300) /* 5 minute rate-limiting */

static void timekeeping_check_update(struct timekeeper *tk, u64 offset)
{

        u64 max_cycles = tk->tkr_mono.clock->max_cycles;
        const char *name = tk->tkr_mono.clock->name;

        if (offset > max_cycles) {
                printk_deferred("WARNING: timekeeping: Cycle offset (%lld) is larger than allowed by the '%s' clock's max_cycles value (%lld): time overflow danger\n",
                                offset, name, max_cycles);
                printk_deferred("         timekeeping: Your kernel is sick, but tries to cope by capping time updates\n");
        } else {
                if (offset > (max_cycles >> 1)) {
                        printk_deferred("INFO: timekeeping: Cycle offset (%lld) is larger than the '%s' clock's 50%% safety margin (%lld)\n",
                                        offset, name, max_cycles >> 1);
                        printk_deferred("      timekeeping: Your kernel is still fine, but is feeling a bit nervous\n");
                }
        }

        if (tk->underflow_seen) {
                if (jiffies - tk->last_warning > WARNING_FREQ) {
                        printk_deferred("WARNING: Underflow in clocksource '%s' observed, time update ignored.\n", name);
                        printk_deferred("         Please report this, consider using a different clocksource, if possible.\n");
                        printk_deferred("         Your kernel is probably still fine.\n");
                        tk->last_warning = jiffies;
                }
                tk->underflow_seen = 0;
        }

> I assume you need very long (like days worth) jump to trigger this bug

Exactly. max_cycles worth (for kvmclock one or two days
vmstop/vmstart was sufficient to trigger the bug).

> and for such case we can either work around it in qemu / kernel 
> or fix it in the guest kernel and I strongly prefer the latter.

Well, what about older kernels? Can't fix those in the guest kernel. 

Moreover:

https://patchwork.kernel.org/project/kvm/patch/20130618233825.GA19042@amt.cnet/

2) Users rely on CLOCK_MONOTONIC to count run time, that is,
time which OS has been in a runnable state (see CLOCK_BOOTTIME).

I think the current 100ms delta (on migration) can be reduced without 
checking the clock delta between source and destination hosts.

So to reiterate: the idea to pass a tuple (tsc, tsc_adjust) is
good because you can fix the issues introduced by writing the values
separately.

However, IMHO the patchset lacks a clear problem (or set of problems) 
that its addressing.

> Thomas, what do you think about it?
> 
> Best regards,
> 	Maxim Levitsky
> 
> > 
> 
> > > +
> > > +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
> 

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