From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 28E44CD4840 for ; Mon, 11 May 2026 16:39:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:MIME-Version:Content-Type: References:In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=Yk5tQrKgGzZrvF5G1Yd6pdLqkkKezaJ+GkE5z2uIwsg=; b=3mVV+ajEweApPbbgOkEx7guAs9 aOJpYxoK9Q/bh2KhZ4hMw1aCsooI9QiLrmMkti+9djLoq+X7CHo7RbBFJP7V5l+MZEC7qkQzJfMlQ BiWhHkchksAs1Z3GNkVZGxnvO2n/zBfaDQRJAlPuv/m7/d/WVdVpx1TUXJnNItN83r/lBxxvy5Czw 4m2GgUaPo+x1TjI5jUuMP35ETOp2KCAzmHVlmdpaIkBXVkcSwOU9QumFI+pb7UMpDl23JDvQoqU03 5XOzNkjmTk0k6JdXycrQDMlSxVqwHFxqWkqHERpXGs+1caPZgm6ocxl/0x6/46Uu+CGmB6Xm9wLdu Epg3MCUw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wMTef-0000000EEjQ-2fSA; Mon, 11 May 2026 16:38:53 +0000 Received: from casper.infradead.org ([2001:8b0:10b:1236::1]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1wMTee-0000000EEjE-3Bqo for linux-arm-kernel@bombadil.infradead.org; Mon, 11 May 2026 16:38:52 +0000 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; 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=Yk5tQrKgGzZrvF5G1Yd6pdLqkkKezaJ+GkE5z2uIwsg=; b=fBbrn49osagTXz3cANaylidQm6 r9hLqXxV8eYeQq4T7kSpLJPeikFNP4XyyCjtIaSjayLHHmmnZ+o0UQg8dmvlD/WwAvdtmB4X13Zdj Y2elbzkz5ETG6z0hgzq1teoqOHUg4qklaBboVszzO7Z52KbkpATgjNclO1Tv7vCPVBcnKwYBB4aHO U9VJ/Bg7sHCzl4m3xqodWLGHy3UWa9qguqzeJDfnqEBn3TcyElHTDbHUgNsZmwkYMDbD3e/SeEqKW wEqNUozYh9+TT8YCyNEWqEXf29g0faLsoMrf3T+obOmumGAYQ0m3CgLMp5mC7CAzm5U1ymwT6vEnD qQLFMkyw==; Received: from 54-240-197-239.amazon.com ([54.240.197.239] helo=edge-cache-179.e-lhr50.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wMTeb-00000008SIG-3nVc; Mon, 11 May 2026 16:38:50 +0000 Message-ID: <57bc082f4824d6114d3156744c25986effc29aca.camel@infradead.org> Subject: Re: [PATCH] Documentation: KVM: Document guest-visible compatibility expectations From: David Woodhouse To: Paolo Bonzini , Jonathan Corbet , Shuah Khan , kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Sean Christopherson , Jim Mattson Cc: Oliver Upton , Joey Gouly , Suzuki K Poulose , Zenghui Yu , Catalin Marinas , Will Deacon , Raghavendra Rao Ananta , Eric Auger , Kees Cook , Arnd Bergmann , Nathan Chancellor , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kselftest@vger.kernel.org Date: Mon, 11 May 2026 17:38:48 +0100 In-Reply-To: <6afc4b95-3c15-4d71-877d-19b84e91ce05@redhat.com> References: <6856b269d2af706eae397e0cf9c1231f89d9a932.camel@infradead.org> <6afc4b95-3c15-4d71-877d-19b84e91ce05@redhat.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-OaKzrBSuo4HYF0RQeXcN" User-Agent: Evolution 3.52.3-0ubuntu1.1 MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org --=-OaKzrBSuo4HYF0RQeXcN Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2026-05-11 at 17:14 +0200, Paolo Bonzini wrote: > On 5/11/26 10:57, David Woodhouse wrote: > > From: David Woodhouse > >=20 > > Document the expectation that KVM maintains guest-visible compatibility > > across host kernel upgrades and rollbacks.=C2=A0 Specifically: > >=20 > > =C2=A0 - State saved/restored via KVM ioctls must be sufficient for liv= e > > =C2=A0=C2=A0=C2=A0 migration (and live update) between kernel versions. > >=20 > > =C2=A0 - Where a new kernel introduces a guest-visible change, it provi= des a > > =C2=A0=C2=A0=C2=A0 mechanism for userspace to select the previous behav= iour. > >=20 > > =C2=A0 - This allows both forward migration (upgrade) and backward migr= ation > > =C2=A0=C2=A0=C2=A0 (rollback) of guests. > >=20 > > These expectations have been implicitly required on x86 but were not > > explicitly documented. Harmonise the expectations across all of KVM. >=20 > One big part of achieving this on x86 is the handling of CPUID.=C2=A0 Des= pite=20 > all the mess that KVM_SET_CPUID2 is (and sometimes the underlying=20 > architecture too, as Jim Mattson would certainly agree), KVM is=20 > generally able to provide a consistent view of its configuration to the= =20 > guest.=C2=A0 This doesn't quite extend to compatibility across vendors, b= ut=20 > it does work across processor generations from either Intel or AMD. Right. For x86 this is largely covered by CPUID. If you launch a guest on a new kernel, using the same CPUID bits as an older kernel, then your guest will mostly not see anything new. And will be migratable to that older kernel without issue. Not *everything* is in CPUID; one recent exception that comes to mind is the SUPPRESS_EOI_BROADCAST quirk. But on x86 we preserve the existing behaviour of older kernels =E2=80=94 even when that behaviour does= n't make much sense, as with SUPPRESS_EOI_BROADCAST where older KVM would *advertise* the feature, but not actually *implement* it. Nevertheless, that remains the default behaviour of future kernels unless userspace explicitly opts in to fully enable (or disable) the feature. But this documentation update isn't even asking for that compatible-by- default behaviour, even though that is the right thing to do. It's only asking that it be *possible* to reinstate the old behaviour, for userspace that *knows* about the change and explicitly wants to go back to the old way to remain compatible. And sadly, KVM/arm64 doesn't even meet *that* low bar. --=-OaKzrBSuo4HYF0RQeXcN 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 aDJAwtycb8yXtILj+TvfhLFhafxroXrflspavejQkEiHjNjtHnwbZ+o43g0/yxjwnarGI3kgcak7 nnI9/8Lqpq79tLHYwLajotwLiGTB71AGN5xK+tzB+D4eN9lXayrjcszgbOv2ZCgzExQUAIt98mre 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