netdev - Re: [PATCH 1/2] ptp: vmclock: add vm generation counter

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Message-ID: <c3bfbf5e9cfaad185a65a64c6a40aaf4bcb69dcd.camel@infradead.org>
Date: Fri, 28 Nov 2025 12:36:06 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: "Chalios, Babis" <bchalios@...zon.es>, "richardcochran@...il.com"
 <richardcochran@...il.com>, "andrew+netdev@...n.ch"
 <andrew+netdev@...n.ch>,  "davem@...emloft.net" <davem@...emloft.net>,
 "edumazet@...gle.com" <edumazet@...gle.com>,  "kuba@...nel.org"
 <kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>, 
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc: "Graf (AWS), Alexander" <graf@...zon.de>, "mzxreary@...inter.de"
	 <mzxreary@...inter.de>
Subject: Re: [PATCH 1/2] ptp: vmclock: add vm generation counter

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@...zon.es>

Reviewed-by: David Woodhouse <dwmw@...zon.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__ */


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