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 A1082371071; Thu, 9 Apr 2026 21:12:25 +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=1775769150; cv=none; b=JzFGwQIPdqUApflnGNxhmlTqEezYZAZT61gelBVJi/1cWVj9KKLCzv69h+VtPe1gndguz5sSHtPMsjIRvIXnEN6ehpEfwRhNMFQH6G0aVzpc01LmZuNKQJyb9fqo5Ce3rov32+s0VBz8VpcRLakAO4KDOS3m9xKmJGiq1yznltY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775769150; c=relaxed/simple; bh=W+QVhhRcZ+wnz8UL9kN1R3TtbKrhNjSuImzbBc7OTyc=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=I0cV0Fwfjh2EatMkmwGma244OSAF+mkO4H4fZNxZpLsl2VcOxfBzV838mcqr8O6xMuX8HOdAHyW34hVwcpydEWuRlwygNnSAEPDV92cszqlfjumzYAnI3VnQaXCjicZ10eLYcuPAvj9F+Bserzm3DS6ScKeHD9FF6TWBgOiG7pw= 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=WzZmKf6c; 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="WzZmKf6c" 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=W+QVhhRcZ+wnz8UL9kN1R3TtbKrhNjSuImzbBc7OTyc=; b=WzZmKf6cTDfScK1gk+1acnZhlr G93RvaO6YUmLTxRxin5L8LZ2vp3pz9jHGvpTVk5E47nExzIAehMSGmWF4yAVB5M6I0KG0HVTC7I93 +tIFzTlJYrx4RBbtUqxfQdTCk5T2F8xu9fZu61c9SJLiEBniTZhRJ+UbBPPCjWNPxXxkO0hz0mErZ bon9KwxkM46pfEdWd/kQUN4urdDsp67jRy84wnZdmmu/C8+FFM6Nt4kXbDsR/N7DSdXr4gT2NLpV0 q2CQgol/+K0gn2nYY3FLFKdlxC7OqpO41PkjsN1VwML4MsIqcbZ6q9/iXgwZBSmxgRj+dyxYv0TdH 45y8tHsw==; Received: from [172.31.31.148] (helo=u09cd745991455d.lumleys.internal) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1wAweG-0000000CPPo-0dZe; Thu, 09 Apr 2026 21:11:34 +0000 Message-ID: <65b108f99ff0bcc54609a37ef5c9ce07a72e4129.camel@infradead.org> Subject: Re: [PATCH] KVM: arm64: Add KVM_CAP_ARM_NATIVE_CACHE_CONFIG vcpu capability From: David Woodhouse To: Marc Zyngier Cc: "Gutierrez Cantu, Bernardo" , alexandru.elisei@arm.com, alyssa@rosenzweig.io, asahi@lists.linux.dev, broonie@kernel.org, catalin.marinas@arm.com, james.morse@arm.com, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, marcan@marcan.st, mathieu.poirier@linaro.org, oliver.upton@linux.dev, suzuki.poulose@arm.com, sven@svenpeter.dev, will@kernel.org Date: Thu, 09 Apr 2026 22:10:47 +0100 In-Reply-To: <86qzoo2eba.wl-maz@kernel.org> References: <7fb7b823c68e04321eb532a5b8ae21a818d4926d.camel@infradead.org> <86se942hbv.wl-maz@kernel.org> <584931abfa8c6f6c9c8a1379d26ef6beb58db1cd.camel@infradead.org> <86qzoo2eba.wl-maz@kernel.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-AMcz7QLTiE7nyEtLfOUk" 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 desiato.infradead.org. See http://www.infradead.org/rpr.html --=-AMcz7QLTiE7nyEtLfOUk Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2026-04-09 at 19:12 +0100, Marc Zyngier wrote: >=20 > That only means you are doing a pretty bad job at supporting > guests. And yes, this is an issue for anything that expects to see > something meaningful in CCSIDR[]. The fact that none of your guests > hit that problem only means you're lacking coverage. I think we just have a different idea of what it means to do a good job of supporting guests. For me, the most important thing is never to randomly change stuff underneath a running guest, even if it's a "bug fix". Even fixing an egregious and obvious bug, like fixing the reported serial port type from the legacy 16550 to ACPI_DBG2_16550_WITH_GAS turned out to break FreeBSD, to pick just one recent example. Guest operating systems will often work with the environment they actually *had* already, regardless of the actual specifications or common sense. We don't have the luxury of just changing things underneath them and then saying "haha, well that should never have worked so screw you". Moving to the fixed model is obviously the plan, but it has to be done carefully and starting with new launches and new instance types and a rollback plan, not just randomly inflicted as an unexpected side-effect of a kernel upgrade. > From what I can read, anything from Neoverse V1 is affected. >=20 > >=20 > > This isn't about introducing *new* behaviour; it's about allowing the > > existing established behaviour to be maintained so that we can have a > > *managed* transition to the new model (for new launches) rather than an > > unconditional uncontrolled change as the kernel gets upgraded. >=20 > Then fully implement "show me the cache hierarchy", read it out, and > write it back with whatever level of brokenness you intend to inflict > on your guests. Again, *compatibility* is the point here, not brokenness. > But I'm not reintroducing this particular bug. Fair enough. It's not like the x86 zoo; there are only a certain number of existing CPUs that I care about, that I need to enumerate. I can just hard-code the values for all of those in the VMM =E2=80=94 the kernel = does let me override the errantly-changed defaults, as the commit message said. --=-AMcz7QLTiE7nyEtLfOUk 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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