On Wed, Oct 14, 2009 at 03:53:27PM -0300, Marcelo Tosatti wrote:
> On Wed, Oct 14, 2009 at 10:47:46AM -0400, Glauber Costa wrote:
> > When we migrate a kvm guest that uses pvclock between two hosts, we may
> > suffer a large skew. This is because there can be significant differences
> > between the monotonic clock of the hosts involved. When a new host with
> > a much larger monotonic time starts running the guest, the view of time
> > will be significantly impacted.
> >
> > Situation is much worse when we do the opposite, and migrate to a host with
> > a smaller monotonic clock.
> >
> > This proposed ioctl will allow userspace to inform us what is the monotonic
> > clock value in the source host, so we can keep the time skew short, and
> > more importantly, never goes backwards. Userspace may also need to trigger
> > the current data, since from the first migration onwards, it won't be
> > reflected by a simple call to clock_gettime() anymore.
> >
> > [ v2: uses a struct with a padding ]
> > [ v3: provide an ioctl to get clock data too ]
> > [ v4: used fixed-width signed type for delta ]
> >
> > Signed-off-by: Glauber Costa <glommer@xxxxxxxxxx>
> > ---
> > arch/x86/include/asm/kvm_host.h | 1 +
> > arch/x86/kvm/x86.c | 35 ++++++++++++++++++++++++++++++++++-
> > include/linux/kvm.h | 7 +++++++
> > 3 files changed, 42 insertions(+), 1 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 179a919..c9b0d9f 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -410,6 +410,7 @@ struct kvm_arch{
> >
> > unsigned long irq_sources_bitmap;
> > u64 vm_init_tsc;
> > + s64 kvmclock_offset;
> > };
> >
> > struct kvm_vm_stat {
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 9601bc6..09f31e2 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -699,7 +699,8 @@ static void kvm_write_guest_time(struct kvm_vcpu *v)
> > /* With all the info we got, fill in the values */
> >
> > vcpu->hv_clock.system_time = ts.tv_nsec +
> > - (NSEC_PER_SEC * (u64)ts.tv_sec);
> > + (NSEC_PER_SEC * (u64)ts.tv_sec) + v->kvm->arch.kvmclock_offset;
> > +
> > /*
> > * The interface expects us to write an even number signaling that the
> > * update is finished. Since the guest won't see the intermediate
> > @@ -2441,6 +2442,38 @@ long kvm_arch_vm_ioctl(struct file *filp,
> > r = 0;
> > break;
> > }
> > + case KVM_SET_CLOCK: {
> > + struct timespec now;
> > + struct kvm_clock_data user_ns;
> > + u64 now_ns;
> > + s64 delta;
> > +
> > + r = -EFAULT;
>
> Extra space :)
want me to send a new because of that?
>
> > #define KVM_CREATE_PIT2 _IOW(KVMIO, 0x77, struct kvm_pit_config)
> > #define KVM_SET_BOOT_CPU_ID _IO(KVMIO, 0x78)
> > #define KVM_IOEVENTFD _IOW(KVMIO, 0x79, struct kvm_ioeventfd)
> > +#define KVM_SET_CLOCK _IOW(KVMIO, 0x7a, struct kvm_clock_data)
> > +#define KVM_GET_CLOCK _IOW(KVMIO, 0x7b, struct kvm_clock_data)
> _IOR
>
> Otherwise looks fine, please send the userspace changes together.
Note that this changed quite a while in the process already. It only makes sense to implement
userspace once this is commited, IMHO.
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