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Date: Tue, 08 Dec 2020 16:50:53 +0200
From: Maxim Levitsky <mlevitsk@...hat.com>
To: Marcelo Tosatti <mtosatti@...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 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.
>
> > +
> > +
> > +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?
I assume you need very long (like days worth) jump to trigger this 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.
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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