Similar to live migration, loading a VM from some saved state (aka
snapshot) is also an event that calls for clock adjustments in the
guest. However, guests might want to take more actions as a response to
such events, e.g. as discarding UUIDs, resetting network connections,
reseeding entropy pools, etc. These are actions that guests don't
typically take during live migration, so add a new field in the
vmclock_abi called vm_generation_counter which informs the guest about
such events.

Hypervisor advertises support for vm_generation_counter through the
VMCLOCK_FLAG_VM_GEN_COUNTER_PRESENT flag. Users need to check the
presence of this bit in vmclock_abi flags field before using this flag.

Signed-off-by: Babis Chalios <bchalios@amazon.es>
---
 include/uapi/linux/vmclock-abi.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/include/uapi/linux/vmclock-abi.h b/include/uapi/linux/vmclock-abi.h
index 2d99b29ac44a..937fe00e4f33 100644
--- a/include/uapi/linux/vmclock-abi.h
+++ b/include/uapi/linux/vmclock-abi.h
@@ -115,6 +115,12 @@ struct vmclock_abi {
 	 * bit again after the update, using the about-to-be-valid fields.
 	 */
 #define VMCLOCK_FLAG_TIME_MONOTONIC		(1 << 7)
+	/*
+	 * If the VM_GEN_COUNTER_PRESENT flag is set, the hypervisor will
+	 * bump the vm_generation_counter field every time the guest is
+	 * loaded from some save state (restored from a snapshot).
+	 */
+#define VMCLOCK_FLAG_VM_GEN_COUNTER_PRESENT     (1 << 8)
 
 	__u8 pad[2];
 	__u8 clock_status;
@@ -177,6 +183,15 @@ struct vmclock_abi {
 	__le64 time_frac_sec;		/* Units of 1/2^64 of a second */
 	__le64 time_esterror_nanosec;
 	__le64 time_maxerror_nanosec;
+
+	/*
+	 * This field changes to another non-repeating value when the guest
+	 * has been loaded from a snapshot. In addition to handling a
+	 * disruption in time (which will also be signalled through the
+	 * disruption_marker field), a guest may wish to discard UUIDs,
+	 * reset network connections, reseed entropy, etc.
+	 */
+	__le64 vm_generation_counter;
 };
 
 #endif /*  __VMCLOCK_ABI_H__ */
-- 
2.34.1

On Thu, 2025-11-27 at 10:32 +0000, Chalios, Babis wrote:
> Similar to live migration, loading a VM from some saved state (aka
> snapshot) is also an event that calls for clock adjustments in the
> guest. However, guests might want to take more actions as a response to
> such events, e.g. as discarding UUIDs, resetting network connections,
> reseeding entropy pools, etc. These are actions that guests don't
> typically take during live migration, so add a new field in the
> vmclock_abi called vm_generation_counter which informs the guest about
> such events.
> 
> Hypervisor advertises support for vm_generation_counter through the
> VMCLOCK_FLAG_VM_GEN_COUNTER_PRESENT flag. Users need to check the
> presence of this bit in vmclock_abi flags field before using this flag.
> 
> Signed-off-by: Babis Chalios <bchalios@amazon.es>

Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>

We are writing up a formal definition of the VMClock device which
includes these updates, and I believe the QEMU patches are to follow.

I've put an early draft at https://david.woodhou.se/VMClock.pdf

> ---
>  include/uapi/linux/vmclock-abi.h | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/include/uapi/linux/vmclock-abi.h b/include/uapi/linux/vmclock-abi.h
> index 2d99b29ac44a..937fe00e4f33 100644
> --- a/include/uapi/linux/vmclock-abi.h
> +++ b/include/uapi/linux/vmclock-abi.h
> @@ -115,6 +115,12 @@ struct vmclock_abi {
>  	 * bit again after the update, using the about-to-be-valid fields.
>  	 */
>  #define VMCLOCK_FLAG_TIME_MONOTONIC		(1 << 7)
> +	/*
> +	 * If the VM_GEN_COUNTER_PRESENT flag is set, the hypervisor will
> +	 * bump the vm_generation_counter field every time the guest is
> +	 * loaded from some save state (restored from a snapshot).
> +	 */
> +#define VMCLOCK_FLAG_VM_GEN_COUNTER_PRESENT     (1 << 8)
>  
>  	__u8 pad[2];
>  	__u8 clock_status;
> @@ -177,6 +183,15 @@ struct vmclock_abi {
>  	__le64 time_frac_sec;		/* Units of 1/2^64 of a second */
>  	__le64 time_esterror_nanosec;
>  	__le64 time_maxerror_nanosec;
> +
> +	/*
> +	 * This field changes to another non-repeating value when the guest
> +	 * has been loaded from a snapshot. In addition to handling a
> +	 * disruption in time (which will also be signalled through the
> +	 * disruption_marker field), a guest may wish to discard UUIDs,
> +	 * reset network connections, reseed entropy, etc.
> +	 */
> +	__le64 vm_generation_counter;
>  };
>  
>  #endif /*  __VMCLOCK_ABI_H__ */