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KVM: x86: Expose TSC offset controls to userspace

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To date, VMM-directed TSC synchronization and migration has been a bit
messy. KVM has some baked-in heuristics around TSC writes to infer if
the VMM is attempting to synchronize. This is problematic, as it depends
on host userspace writing to the guest's TSC within 1 second of the last
write.

A much cleaner approach to configuring the guest's views of the TSC is to
simply migrate the TSC offset for every vCPU. Offsets are idempotent,
and thus not subject to change depending on when the VMM actually
reads/writes values from/to KVM. The VMM can then read the TSC once with
KVM_GET_CLOCK to capture a (realtime, host_tsc) pair at the instant when
the guest is paused.

Cc: David Matlack <dmatlack@google.com>
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210916181538.968978-8-oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Oliver Upton 2021-09-16 18:15:38 +00:00 committed by Paolo Bonzini
parent 58d4277be9
commit 828ca89628
4 changed files with 178 additions and 0 deletions
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57
Documentation/virt/kvm/devices/vcpu.rst
View File

@ -161,3 +161,60 @@ Specifies the base address of the stolen time structure for this VCPU. The
base address must be 64 byte aligned and exist within a valid guest memory
region. See Documentation/virt/kvm/arm/pvtime.rst for more information
including the layout of the stolen time structure.
4. GROUP: KVM_VCPU_TSC_CTRL
===========================
:Architectures: x86
4.1 ATTRIBUTE: KVM_VCPU_TSC_OFFSET
:Parameters: 64-bit unsigned TSC offset
Returns:
======= ======================================
-EFAULT Error reading/writing the provided
parameter address.
-ENXIO Attribute not supported
======= ======================================
Specifies the guest's TSC offset relative to the host's TSC. The guest's
TSC is then derived by the following equation:
guest_tsc = host_tsc + KVM_VCPU_TSC_OFFSET
This attribute is useful for the precise migration of a guest's TSC. The
following describes a possible algorithm to use for the migration of a
guest's TSC:
From the source VMM process:
1. Invoke the KVM_GET_CLOCK ioctl to record the host TSC (t_0),
kvmclock nanoseconds (k_0), and realtime nanoseconds (r_0).
2. Read the KVM_VCPU_TSC_OFFSET attribute for every vCPU to record the
guest TSC offset (off_n).
3. Invoke the KVM_GET_TSC_KHZ ioctl to record the frequency of the
guest's TSC (freq).
From the destination VMM process:
4. Invoke the KVM_SET_CLOCK ioctl, providing the kvmclock nanoseconds
(k_0) and realtime nanoseconds (r_0) in their respective fields.
Ensure that the KVM_CLOCK_REALTIME flag is set in the provided
structure. KVM will advance the VM's kvmclock to account for elapsed
time since recording the clock values.
5. Invoke the KVM_GET_CLOCK ioctl to record the host TSC (t_1) and
kvmclock nanoseconds (k_1).
6. Adjust the guest TSC offsets for every vCPU to account for (1) time
elapsed since recording state and (2) difference in TSCs between the
source and destination machine:
new_off_n = t_0 + off_n + (k_1 - k_0) * freq - t_1
7. Write the KVM_VCPU_TSC_OFFSET attribute for every vCPU with the
respective value derived in the previous step.

1
arch/x86/include/asm/kvm_host.h
View File

@ -1095,6 +1095,7 @@ struct kvm_arch {
u64 last_tsc_nsec;
u64 last_tsc_write;
u32 last_tsc_khz;
u64 last_tsc_offset;
u64 cur_tsc_nsec;
u64 cur_tsc_write;
u64 cur_tsc_offset;

4
arch/x86/include/uapi/asm/kvm.h
View File

@ -504,4 +504,8 @@ struct kvm_pmu_event_filter {
#define KVM_PMU_EVENT_ALLOW 0
#define KVM_PMU_EVENT_DENY 1
/* for KVM_{GET,SET,HAS}_DEVICE_ATTR */
#define KVM_VCPU_TSC_CTRL 0 /* control group for the timestamp counter (TSC) */
#define KVM_VCPU_TSC_OFFSET 0 /* attribute for the TSC offset */
#endif /* _ASM_X86_KVM_H */

116
arch/x86/kvm/x86.c
View File

@ -2454,6 +2454,7 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc,
kvm->arch.last_tsc_nsec = ns;
kvm->arch.last_tsc_write = tsc;
kvm->arch.last_tsc_khz = vcpu->arch.virtual_tsc_khz;
kvm->arch.last_tsc_offset = offset;
vcpu->arch.last_guest_tsc = tsc;
@ -4054,6 +4055,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
case KVM_CAP_VM_COPY_ENC_CONTEXT_FROM:
case KVM_CAP_SREGS2:
case KVM_CAP_EXIT_ON_EMULATION_FAILURE:
case KVM_CAP_VCPU_ATTRIBUTES:
r = 1;
break;
case KVM_CAP_EXIT_HYPERCALL:
@ -4918,6 +4920,115 @@ static int kvm_set_guest_paused(struct kvm_vcpu *vcpu)
return 0;
}
static int kvm_arch_tsc_has_attr(struct kvm_vcpu *vcpu,
struct kvm_device_attr *attr)
{
int r;
switch (attr->attr) {
case KVM_VCPU_TSC_OFFSET:
r = 0;
break;
default:
r = -ENXIO;
}
return r;
}
static int kvm_arch_tsc_get_attr(struct kvm_vcpu *vcpu,
struct kvm_device_attr *attr)
{
u64 __user *uaddr = (u64 __user *)(unsigned long)attr->addr;
int r;
if ((u64)(unsigned long)uaddr != attr->addr)
return -EFAULT;
switch (attr->attr) {
case KVM_VCPU_TSC_OFFSET:
r = -EFAULT;
if (put_user(vcpu->arch.l1_tsc_offset, uaddr))
break;
r = 0;
break;
default:
r = -ENXIO;
}
return r;
}
static int kvm_arch_tsc_set_attr(struct kvm_vcpu *vcpu,
struct kvm_device_attr *attr)
{
u64 __user *uaddr = (u64 __user *)(unsigned long)attr->addr;
struct kvm *kvm = vcpu->kvm;
int r;
if ((u64)(unsigned long)uaddr != attr->addr)
return -EFAULT;
switch (attr->attr) {
case KVM_VCPU_TSC_OFFSET: {
u64 offset, tsc, ns;
unsigned long flags;
bool matched;
r = -EFAULT;
if (get_user(offset, uaddr))
break;
raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
matched = (vcpu->arch.virtual_tsc_khz &&
kvm->arch.last_tsc_khz == vcpu->arch.virtual_tsc_khz &&
kvm->arch.last_tsc_offset == offset);
tsc = kvm_scale_tsc(vcpu, rdtsc(), vcpu->arch.l1_tsc_scaling_ratio) + offset;
ns = get_kvmclock_base_ns();
__kvm_synchronize_tsc(vcpu, offset, tsc, ns, matched);
raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
r = 0;
break;
}
default:
r = -ENXIO;
}
return r;
}
static int kvm_vcpu_ioctl_device_attr(struct kvm_vcpu *vcpu,
unsigned int ioctl,
void __user *argp)
{
struct kvm_device_attr attr;
int r;
if (copy_from_user(&attr, argp, sizeof(attr)))
return -EFAULT;
if (attr.group != KVM_VCPU_TSC_CTRL)
return -ENXIO;
switch (ioctl) {
case KVM_HAS_DEVICE_ATTR:
r = kvm_arch_tsc_has_attr(vcpu, &attr);
break;
case KVM_GET_DEVICE_ATTR:
r = kvm_arch_tsc_get_attr(vcpu, &attr);
break;
case KVM_SET_DEVICE_ATTR:
r = kvm_arch_tsc_set_attr(vcpu, &attr);
break;
}
return r;
}
static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
struct kvm_enable_cap *cap)
{
@ -5372,6 +5483,11 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
r = __set_sregs2(vcpu, u.sregs2);
break;
}
case KVM_HAS_DEVICE_ATTR:
case KVM_GET_DEVICE_ATTR:
case KVM_SET_DEVICE_ATTR:
r = kvm_vcpu_ioctl_device_attr(vcpu, ioctl, argp);
break;
default:
r = -EINVAL;
}