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 CA15917BCA; Thu, 9 Apr 2026 14:52:06 +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=1775746331; cv=none; b=swy4ToQGpHC6jgYB1X9/8DF3oYaMS64n3rTZQVCLMM/K/+/DGnG+X8EpPG6TgsrPU5JKts4ZpwBoAd4nw+PEGaZNtR56FNgCEJJiDYD9p8HjxmOqV/crYrazlvgMxgKfh5TNLL3Lz9Vjjj+mgMWY8QCx6kch8VKwjWrZZa86HpY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775746331; c=relaxed/simple; bh=hNFRjrU42Z6SYVO+w/DOnUGPmFLCpSMQlxr2/OLaq14=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=FUysoPE5AazCTTbG01iwzqNVmtC19u7GRwI5Lunr+xZu2Rjp56ShY+rCQx1kbSIxuPeAVWe10F+FYfqYxEKOtnHyMu0BMR7+OTNwYgFiaFnVYvQHM+D0O1VmL5ksjZbMFJ7rd/MijRv1YmfO/Kn3EdrDTfQWgbi0HFQHXO9z3pc= 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=DHMRrDUS; 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="DHMRrDUS" 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=hNFRjrU42Z6SYVO+w/DOnUGPmFLCpSMQlxr2/OLaq14=; b=DHMRrDUS+PFfnPeJelsWuX1O4k Io4bggwcxcvyrMwvEB/5TpkaVw3FXdHZs7VNG2KlwHaFTQXQu9bV1cv41Fzd0+MunOkwVGAlOzdAO 9wLvSBY8TigOUSCNC5n+5DtWzbcmElytlOtzzvCitNrRDEYDKp6JwvtSy8646oyxEyM1S/E6tDYu2 vT4EfEoeIiMMwZpYlbyyLhN1CW/AjLU0kAXQfTqTYwZqnFppFD3p5HQYbsmEzLpUqyhEUA5RjWtl9 w1zSquJp0m9zBd+mYXUOnUhR7v5HKilFPZfPVigeCzzvPIXC/CO8LRfEVtFaafzudzvUribjQ0Efo waYVLsiA==; 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 1wAqjf-0000000BvFm-3wLU; Thu, 09 Apr 2026 14:52:00 +0000 Message-ID: <254ca48a67779ccf9b9f60e2bb5796a305c03f95.camel@infradead.org> Subject: Re: [PATCH v7 7/7] KVM: arm64: Normalize cache configuration From: David Woodhouse To: Marc Zyngier Cc: akihiko.odaki@daynix.com, "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.cs.columbia.edu, 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 15:51:59 +0100 In-Reply-To: <86wlyg2r3i.wl-maz@kernel.org> References: <20230112023852.42012-8-akihiko.odaki@daynix.com> <86wlyg2r3i.wl-maz@kernel.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-kWqLsqUWPHxL54CrnK2x" 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 --=-kWqLsqUWPHxL54CrnK2x Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2026-04-09 at 14:36 +0100, Marc Zyngier wrote: > On Thu, 09 Apr 2026 13:25:24 +0100, > David Woodhouse wrote: > >=20 > > On Thu, 12 Jan 2023 at 11:38:52 +0900, Akihiko Odaki wrote: > > > Before this change, the cache configuration of the physical CPU was > > > exposed to vcpus. This is problematic because the cache configuration= a > > > vcpu sees varies when it migrates between vcpus with different cache > > > configurations. > > >=20 > > > Fabricate cache configuration from the sanitized value, which holds t= he > > > CTR_EL0 value the userspace sees regardless of which physical CPU it > > > resides on. > > >=20 > > > CLIDR_EL1 and CCSIDR_EL1 are now writable from the userspace so that > > > the VMM can restore the values saved with the old kernel. > >=20 > > (commit 7af0c2534f4c5) > >=20 > > How does the VMM set the values that the old kernel would have set? >=20 > By reading them at the source? Yes, but the VMM in EL0 *can't* read the source, can it? > > Let's say we're deploying a kernel with this change for the first time, > > and we need to ensure that we provide a consistent environment to > > guests, which can be live migrated back to an older host. >=20 > We have never guaranteed host downgrade. It almost never works. Huh? Host downgrade absolutely *does* work; KVM on Arm isn't a toy. We'd never be able to ship a new kernel if we didn't know we could roll it *back* if we needed to. I *know* that you know perfectly well that we actively test host downgrades prior to every rollout of a new kernel. So I'm a little confused about what you're trying to say here, Marc. > > So for new launches, we need to provide the values that the old kernel > > *would* have provided to the guest. A new launch isn't a migration; > > there are no "values saved with the old kernel". >=20 > And you can provide these values. If I know them, sure. But I don't, because: > > Userspace can't read the CLIDR_EL1 and CCSIDR_EL1 registers directly, > > and AFAICT not everything we need to reconstitute them is in sysfs. How > > is this supposed to work? > >=20 > > Shouldn't this change have been made as a capability that the VMM can > > explicitly opt in or out of? Environments that don't do cross-CPU > > migration absolutely don't care about, and actively don't *want*, the > > sanitisation that this commit inflicted on us, surely? >=20 > I don't think a capability buys you anything. You want to expose > something to the guest? Make it so. You are in the favourable > situation to completely own the HW and the VMM. Now that the values are writable, but userspace can't easily see *what* they would have been in the previous kernel. So having the capability to ask KVM to set them to those values seems like it might be useful. > > Am I missing something? >=20 > That you had over 3 years to voice your concern, and did nothing? I was looking more for technical input about how I might determine the original values to retain compatibility... but yes, I have given feedback that merely *reverting* the offending commit in previous kernel upgrades without actually doing anything to fix it properly wasn't the best choice. --=-kWqLsqUWPHxL54CrnK2x Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCD9Aw ggSOMIIDdqADAgECAhAOmiw0ECVD4cWj5DqVrT9PMA0GCSqGSIb3DQEBCwUAMGUxCzAJBgNVBAYT AlVTMRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5jb20xJDAi BgNVBAMTG0RpZ2lDZXJ0IEFzc3VyZWQgSUQgUm9vdCBDQTAeFw0yNDAxMzAwMDAwMDBaFw0zMTEx MDkyMzU5NTlaMEExCzAJBgNVBAYTAkFVMRAwDgYDVQQKEwdWZXJva2V5MSAwHgYDVQQDExdWZXJv a2V5IFNlY3VyZSBFbWFpbCBHMjCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMjvgLKj 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