From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (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 C110A409615; Tue, 19 May 2026 13:24:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779197072; cv=none; b=hX6vgoJrTItaUTIWmzpoeipthUDmh/eqb2jTaLHiwEbfMzfHw9Eex06VgVFpbxvUGvQdUSiWydqMMqYyPYgpNhqM1/VwFS8zMXTorJ2ER2ZOOrnOgnizwV4WgEFWci+KBrazr6SaR+TzTc5t57sjBns1a4XUZSQhhyVhw9+2oo4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779197072; c=relaxed/simple; bh=sr/q5W97G/GgreOLLhjVOht+bOtir9rNZPxS86m4SlA=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=kO24NURvv37jisNbtrHPM6bhi7VqbPy76SBxbo0ggjwVcq8ldlz9Dl0J+dLqfHL3ACux+7CQUmDR5sfsDmoNkUixX2K+9GspEZvqFQrEGye0kUP0qDmF6fJ78KTKvavdd3A17NKgs6Zq5PGdPJmVslIXeKg5OIma5zcfFOBLEdI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=casper.srs.infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=QXwOGOsi; arc=none smtp.client-ip=90.155.50.34 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=casper.srs.infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="QXwOGOsi" 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=sr/q5W97G/GgreOLLhjVOht+bOtir9rNZPxS86m4SlA=; b=QXwOGOsikJnjZClBeiKtd6LG1b wv8w5Oxzb7HhDgWaLqk0sKyDSG0IUSz9gBIkVUe1tdzyLkbuMlk+WzcSsnHgSQuOhQW/IVLbZD0l/ NmvxFoyi1VDa86vmf2nA7JkYnR/MAWyFw8dx0xISVgI0K7JbH5MwDmB0NMMtgh9BlLyRTYRMWI+rD 4eh49nv03fDUilF/Ye8eooS7HPvhhpCyJErMa6FzsZwc7JwH06qmzY1AWI+cpjP8lwrtIjqVSsw3c vHkyVH3Z9HuC1iiR4cnH4Xi5y/97V6SSxqIfsuXAESKQl4PgEjla6E3BBAv+q4sSlq7EHkVy3kG1G xWCK01bg==; Received: from 54-240-197-233.amazon.com ([54.240.197.233] helo=u09cd745991455d.ant.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wPKQo-00000005wmo-3B2S; Tue, 19 May 2026 13:24:22 +0000 Message-ID: Subject: Re: [PATCH] Documentation: KVM: Document guest-visible compatibility expectations From: David Woodhouse To: Marc Zyngier , Paolo Bonzini Cc: Will Deacon , Jonathan Corbet , Shuah Khan , kvm , Linux Doc Mailing List , "Kernel Mailing List, Linux" , Sean Christopherson , Jim Mattson , Oliver Upton , Joey Gouly , Suzuki K Poulose , Zenghui Yu , Catalin Marinas , Raghavendra Rao Ananta , Eric Auger , Kees Cook , Arnd Bergmann , Nathan Chancellor , linux-arm-kernel , kvmarm@lists.linux.dev, linux-kselftest Date: Tue, 19 May 2026 14:24:21 +0100 In-Reply-To: <86pl2rwoat.wl-maz@kernel.org> References: <6856b269d2af706eae397e0cf9c1231f89d9a932.camel@infradead.org> <6afc4b95-3c15-4d71-877d-19b84e91ce05@redhat.com> <57bc082f4824d6114d3156744c25986effc29aca.camel@infradead.org> <86h5obya2r.wl-maz@kernel.org> <48b06e5655d56ff6eda30e563b34894fa0eb2f07.camel@infradead.org> <3f9d731c3d26b0367600f1069e6425099bc34eac.camel@infradead.org> <86qzn7wp3y.wl-maz@kernel.org> <86pl2rwoat.wl-maz@kernel.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-zKDurwcbWVveeC9GNtML" User-Agent: Evolution 3.52.3-0ubuntu1.1 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 casper.infradead.org. See http://www.infradead.org/rpr.html --=-zKDurwcbWVveeC9GNtML Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue, 2026-05-19 at 13:56 +0100, Marc Zyngier wrote: > On Tue, 19 May 2026 13:38:57 +0100, > Marc Zyngier wrote: > >=20 > > As I said before, I'd be OK with something that would restore IIDR to > > REV1. But not something that actively breaks the GIC emulation by > > reintroducing a bug. That's, by construction, dead code that will only > > bitrot, because there is no SW that can make use of this nonsense. >=20 > I will also add that if we make it a policy to preserve buggy > behaviours that the guest cannot be relying on, then I question > whether we should be fixing anything at all. I think we just have a different understanding of what it practically means to have behaviour "that the guest cannot be relying on", as in the examples I just described for the IIDR issue. > For example, 6.19 fixed a totally buggy behaviour where a guest > couldn't not have more than (on most HW) 4 interrupts in flight at any > given time. This was obviously totally bogus, and this was fixed > unconditionally, as legitimate guests could experience gold-platted > lock-ups. And marked with a Fixes: tag and backported to stable, one presumes? I'm confused that you think this is relevant. Can you contrive a situation where a guest actually relied on this bug and *survived*, like the situations I just explained for the IIDR issue? You can nit-pick my hypotheticals as unlikely =E2=80=94 and they are. But i= f we always just YOLO it and change guest-visible behaviour on the basis that it's "unlikely" to break anyone, and there are many such changes in a given kernel deployment (e.g. from v6.6 to v6.18), then the cumulative probability of being bitten by one of those "unlikely" problems approaches 1. There's a reason we do a *huge* amount of testing of what the guest sees as we move from one kernel to the next, and back again, and endeavour to eliminate all those differences. And once the new kernel *is* deployed and won't be rolled back, of course, all new launches can get the newer behaviour (and the latest version of PSCI, etc...) --=-zKDurwcbWVveeC9GNtML 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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