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 :)
> #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.
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