From: Peter Hornyack <peterhornyack@xxxxxxxxxx>
The KVM_SET_MSR vcpu ioctl has some temporal and value-based heuristics
for determining when userspace is attempting to synchronize TSCs.
Instead of guessing at userspace's intentions in the kernel, directly
expose control of the TSC offset field to userspace such that userspace
may deliberately synchronize the guest TSCs.
Note that TSC offset support is mandatory for KVM on both SVM and VMX.
Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>
Reviewed-by: Peter Shier <pshier@xxxxxxxxxx>
Signed-off-by: Peter Hornyack <peterhornyack@xxxxxxxxxx>
[oupton - expanded docs, fixed KVM master clock interaction]
Signed-off-by: Oliver Upton <oupton@xxxxxxxxxx>
---
Documentation/virt/kvm/api.rst | 31 +++++++++++++++++++++
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/kvm/x86.c | 49 +++++++++++++++++++++++++++++++++
include/uapi/linux/kvm.h | 5 ++++
4 files changed, 86 insertions(+)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 644e5326aa50..7ecef103cc7f 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -4701,6 +4701,37 @@ KVM_PV_VM_VERIFY
KVM is allowed to start protected VCPUs.
+4.126 KVM_GET_TSC_OFFSET
+------------------------
+
+:Capability: KVM_CAP_TSC_OFFSET
+:Architectures: x86
+:Type: vcpu ioctl
+:Parameters: __u64 (out)
+:Returns: 0 on success, < 0 on error
+
+This ioctl gets the TSC offset field. The offset is returned as an
+unsigned value, though it is interpreted as a signed value by hardware.
+
+
+4.127 KVM_SET_TSC_OFFSET
+------------------------
+
+:Capability: KVM_CAP_TSC_OFFSET
+:Architectures: x86
+:Type: vcpu ioctl
+:Parameters: __u64 (in)
+:Returns: 0 on success, < 0 on error
+
+This ioctl sets the TSC offset field. The offset is represented as an
+unsigned value, though it is interpreted as a signed value by hardware.
+The guest's TSC value will change based on the written offset. A
+userspace VMM may use this ioctl to restore the guest's TSC instead of
+KVM_SET_MSRS, thereby avoiding the in-kernel TSC synchronization
+detection. To that end, a userspace VMM may deliberately synchronize
+TSCs by setting the same offset value for all vCPUs.
+
+
5. The kvm_run structure
========================
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 5aaef036627f..0d0e3670fffe 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -920,6 +920,7 @@ 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;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a5c88f747da6..98dbe98874de 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2026,6 +2026,11 @@ u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc)
}
EXPORT_SYMBOL_GPL(kvm_read_l1_tsc);
+static u64 kvm_vcpu_read_tsc_offset(struct kvm_vcpu *vcpu)
+{
+ return vcpu->arch.l1_tsc_offset;
+}
+
static void kvm_vcpu_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
{
vcpu->arch.l1_tsc_offset = offset;
@@ -2063,6 +2068,7 @@ static void kvm_write_tsc_offset(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;
@@ -2160,6 +2166,26 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
EXPORT_SYMBOL_GPL(kvm_write_tsc);
+static void kvm_vcpu_ioctl_set_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
+{
+ struct kvm *kvm = vcpu->kvm;
+ unsigned long flags;
+ bool matched;
+ u64 tsc, ns;
+
+ raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
+
+ matched = (kvm->arch.last_tsc_offset == offset &&
+ vcpu->arch.virtual_tsc_khz &&
+ kvm->arch.last_tsc_khz == vcpu->arch.virtual_tsc_khz);
+
+ tsc = kvm_scale_tsc(vcpu, rdtsc()) + offset;
+ ns = get_kvmclock_base_ns();
+
+ kvm_write_tsc_offset(vcpu, offset, tsc, ns, matched);
+ raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
+}
+
static inline void adjust_tsc_offset_guest(struct kvm_vcpu *vcpu,
s64 adjustment)
{
@@ -3492,6 +3518,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
case KVM_CAP_HYPERV_TIME:
case KVM_CAP_IOAPIC_POLARITY_IGNORED:
case KVM_CAP_TSC_DEADLINE_TIMER:
+ case KVM_CAP_TSC_OFFSET:
case KVM_CAP_DISABLE_QUIRKS:
case KVM_CAP_SET_BOOT_CPU_ID:
case KVM_CAP_SPLIT_IRQCHIP:
@@ -4744,6 +4771,28 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
r = 0;
break;
}
+ case KVM_GET_TSC_OFFSET: {
+ u64 tsc_offset;
+
+ r = -EFAULT;
+ tsc_offset = kvm_vcpu_read_tsc_offset(vcpu);
+ if (copy_to_user(argp, &tsc_offset, sizeof(tsc_offset)))
+ goto out;
+ r = 0;
+ break;
+ }
+ case KVM_SET_TSC_OFFSET: {
+ u64 __user *tsc_offset_arg = argp;
+ u64 tsc_offset;
+
+ r = -EFAULT;
+ if (copy_from_user(&tsc_offset, tsc_offset_arg,
+ sizeof(tsc_offset)))
+ goto out;
+ kvm_vcpu_ioctl_set_tsc_offset(vcpu, tsc_offset);
+ r = 0;
+ break;
+ }
default:
r = -EINVAL;
}
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index ff9b335620d0..41f387ffcd11 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1033,6 +1033,7 @@ struct kvm_ppc_resize_hpt {
#define KVM_CAP_HALT_POLL 182
#define KVM_CAP_ASYNC_PF_INT 183
#define KVM_CAP_LAST_CPU 184
+#define KVM_CAP_TSC_OFFSET 185
#ifdef KVM_CAP_IRQ_ROUTING
@@ -1501,6 +1502,10 @@ struct kvm_enc_region {
#define KVM_S390_NORMAL_RESET _IO(KVMIO, 0xc3)
#define KVM_S390_CLEAR_RESET _IO(KVMIO, 0xc4)
+/* Available with KVM_CAP_TSC_OFFSET */
+#define KVM_GET_TSC_OFFSET _IOR(KVMIO, 0xc5, __u64)
+#define KVM_SET_TSC_OFFSET _IOW(KVMIO, 0xc6, __u64)
+
struct kvm_s390_pv_sec_parm {
__u64 origin;
__u64 length;
--
2.28.0.rc0.142.g3c755180ce-goog
