From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4CFFE2D6E61; Wed, 26 Nov 2025 14:14:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764166479; cv=none; b=mVI10YQ4naN9h8fS8xBui6HowF19/DJMT8T4U1jokInGgJ1Wg5E071/XV9SlLfhBdH0qxDvWoZnWTMiDuG8cMwepBC8u0vHfH4tAjJ6Hv670UDZJce2aH6MikHYa3g6YislyHCS2eNoaLt6pxugExI+xICZfoebpRHEyiwaYQgI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764166479; c=relaxed/simple; bh=xVjTk2jzLbAxXYSZNBJBcZfk3x8gCryYxGjkjkccXXE=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=TgffUXhbktVTwwxiokK2cx+yk7ZbdI6SyiIvVULlQIS++XC/bNly+TIceEy4xGJSDcNH8O+5XubVV3AnhV0QXX2gDMCMO/p5fz/aX6W93KrTRA2BFVfwWzjEwthD5LOB+aLKhUcs+FEPn986Ao05M/glrGRYilS6TZxv9OAnabs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=desiato.srs.infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=LvZ7SMWq; arc=none smtp.client-ip=90.155.92.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=desiato.srs.infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="LvZ7SMWq" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=MIME-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=shJ8INqCd2/yfnuiDjwObFsi6cv+nBH3HMTZig1ESzE=; b=LvZ7SMWqxM+HNf5d6MXxszdtWr d4qcpEcBkzGc48eN7Goq9z6iTRJeUpifTfsFiIldo29+KTsYcVbqjkNSFfU40yRVifLHRQEj1tsO2 v7jF1sHyWGr4eqXrXaukQKp6yJlPzdrhe2sxsA8R20bfV1cTR5Wo/RvB1jGRQwjMOT27QYHb7LUD6 OZES2jkmXNlljA5Q1Gx7KT4YgzI3EFMA8zFVvVhIHudx+oDocE0vJw++BKU3TMsuhSbsrMQeXs3+H hpm1YUlYjOXY7FPtLVDZZmU4/vgqDQVkfNpVuHpos99GvoRTy+DfFYN63knXGvxqRieN/5MXDx8x1 PNfUSA/g==; Received: from [172.31.31.148] (helo=u09cd745991455d.ant.amazon.com) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vOFQH-000000086cM-0SMZ; Wed, 26 Nov 2025 13:19:05 +0000 Message-ID: Subject: Re: [RFC PATCH 1/2] ptp: vmclock: add vm generation counter From: David Woodhouse To: "Chalios, Babis" , "richardcochran@gmail.com" , "andrew+netdev@lunn.ch" , "davem@davemloft.net" , "edumazet@google.com" , "kuba@kernel.org" , "pabeni@redhat.com" , "netdev@vger.kernel.org" , "linux-kernel@vger.kernel.org" Cc: "Graf (AWS), Alexander" , "mzxreary@0pointer.de" Date: Wed, 26 Nov 2025 14:14:27 +0000 In-Reply-To: <20251125153830.11487-2-bchalios@amazon.es> References: <20251125153830.11487-1-bchalios@amazon.es> <20251125153830.11487-2-bchalios@amazon.es> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-PgzzRXrfw4Pk0aNK2tYW" User-Agent: Evolution 3.52.3-0ubuntu1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by desiato.infradead.org. See http://www.infradead.org/rpr.html --=-PgzzRXrfw4Pk0aNK2tYW Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue, 2025-11-25 at 15:38 +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. >=20 > Signed-off-by: Babis Chalios > --- > =C2=A0include/uapi/linux/vmclock-abi.h | 19 +++++++++++++++++++ > =C2=A01 file changed, 19 insertions(+) >=20 > diff --git a/include/uapi/linux/vmclock-abi.h b/include/uapi/linux/vmcloc= k-abi.h > index 2d99b29ac44a..fbf1c5928273 100644 > --- a/include/uapi/linux/vmclock-abi.h > +++ b/include/uapi/linux/vmclock-abi.h > @@ -115,6 +115,12 @@ struct vmclock_abi { > =C2=A0 * bit again after the update, using the about-to-be-valid fields. > =C2=A0 */ > =C2=A0#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=C2=A0=C2=A0=C2=A0=C2=A0 (1 <= < 8) > =C2=A0 > =C2=A0 __u8 pad[2]; > =C2=A0 __u8 clock_status; > @@ -177,6 +183,19 @@ struct vmclock_abi { > =C2=A0 __le64 time_frac_sec; /* Units of 1/2^64 of a second */ > =C2=A0 __le64 time_esterror_nanosec; > =C2=A0 __le64 time_maxerror_nanosec; > + > + /* > + * This field changes to another non-repeating value when the VM > + * is loaded from a snapshot. This event, typically, represents a > + * "jump" forward in time. As a result, in this case as well, the > + * guest needs to discard any calibrarion against external sources. > + * Loading a snapshot in a VM has different semantics than other VM > + * events such as live migration, i.e. apart from re-adjusting guest > + * clocks a guest user space might want to discard UUIDs, reset > + * network connections or reseed entropy, etc. As a result, we > + * use a dedicated marker for such events. > + */ > + __le64 vm_generation_counter; > =C2=A0}; Looks good, thank you. Just a bit of nitpicking about the comment. I really don't like talking about a "jump forward in time". That's what clocks *do*. VMClock tells the guest a relationship between its CPU counter (TSC, arch timer, etc.) and real time. As long as that relationship remains within the error bounds which were previously promised, there is no disruption. It certainly isn't about time jumping *forward*. Let's also make it 100% clear that an implementation must *also* bump the disruption_marker field in this case, not only the vm_generation_counter. How about... "A change in this field indicates that 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 or reseed entropy, etc." --=-PgzzRXrfw4Pk0aNK2tYW Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCD9Aw ggSOMIIDdqADAgECAhAOmiw0ECVD4cWj5DqVrT9PMA0GCSqGSIb3DQEBCwUAMGUxCzAJBgNVBAYT AlVTMRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5jb20xJDAi BgNVBAMTG0RpZ2lDZXJ0IEFzc3VyZWQgSUQgUm9vdCBDQTAeFw0yNDAxMzAwMDAwMDBaFw0zMTEx MDkyMzU5NTlaMEExCzAJBgNVBAYTAkFVMRAwDgYDVQQKEwdWZXJva2V5MSAwHgYDVQQDExdWZXJv a2V5IFNlY3VyZSBFbWFpbCBHMjCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMjvgLKj jfhCFqxYyRiW8g3cNFAvltDbK5AzcOaR7yVzVGadr4YcCVxjKrEJOgi7WEOH8rUgCNB5cTD8N/Et GfZI+LGqSv0YtNa54T9D1AWJy08ZKkWvfGGIXN9UFAPMJ6OLLH/UUEgFa+7KlrEvMUupDFGnnR06 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