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kvm: kvmclock: let KVM_GET_CLOCK return whether the master clock is in use

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Userspace can read the exact value of kvmclock by reading the TSC
and fetching the timekeeping parameters out of guest memory.  This
however is brittle and not necessary anymore with KVM 4.11.  Provide
a mechanism that lets userspace know if the new KVM_GET_CLOCK
semantics are in effect, and---since we are at it---if the clock
is stable across all VCPUs.

Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
This commit is contained in:
Paolo Bonzini
2016-11-09 17:48:15 +01:00
committed by Radim Krčmář
parent 1650b4ebc9
commit e3fd9a93a1
3 changed files with 25 additions and 3 deletions
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arch/x86/kvm/x86.c
+7 -3
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@@ -2610,7 +2610,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
case KVM_CAP_PIT_STATE2:
case KVM_CAP_SET_IDENTITY_MAP_ADDR:
case KVM_CAP_XEN_HVM:
case KVM_CAP_ADJUST_CLOCK:
case KVM_CAP_VCPU_EVENTS:
case KVM_CAP_HYPERV:
case KVM_CAP_HYPERV_VAPIC:
@@ -2637,6 +2636,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
#endif
r = 1;
break;
case KVM_CAP_ADJUST_CLOCK:
r = KVM_CLOCK_TSC_STABLE;
break;
case KVM_CAP_X86_SMM:
/* SMBASE is usually relocated above 1M on modern chipsets,
* and SMM handlers might indeed rely on 4G segment limits,
@@ -4117,9 +4119,11 @@ long kvm_arch_vm_ioctl(struct file *filp,
struct kvm_clock_data user_ns;
u64 now_ns;
now_ns = get_kvmclock_ns(kvm);
local_irq_disable();
now_ns = __get_kvmclock_ns(kvm);
user_ns.clock = now_ns;
user_ns.flags = 0;
user_ns.flags = kvm->arch.use_master_clock ? KVM_CLOCK_TSC_STABLE : 0;
local_irq_enable();
memset(&user_ns.pad, 0, sizeof(user_ns.pad));
r = -EFAULT;