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 15F38441037; Mon, 11 May 2026 17:53:15 +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=1778522000; cv=none; b=QzdD90YJG3cehgVJCOLf1gxEJWCtle7fwledJaKcmH7YXgm8UepaBWYIfX2TlsSXanWc2YO0dXuyLEHZAMJh4ebI/T3KvsXTrPXu0nUbKMDUAC5fSaHYZ08gWGGbBbpQOOCldCXLphNA1hjvTjsOWc3NtMIWWDp2kBopHCW38iQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778522000; c=relaxed/simple; bh=bEerhIEewmfT/YDQQReJK1JTsyW1j7y6PqxaMsqRbsk=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=T1SeZDat9kZ6Y+m1RdeWlfUbxY5zFbsOsxU6iPZcZN5U18BPClbDQNm0+7xGGVppTy5Vyo1KjIpoUI0D0bxylaYTWQLDm73r7j1bLOsmG4lXHYIkFeSmnVuUmmbyhNFCns5r6new5vVfNNmlrGmb6loA0t2lxDKfq/5ZS7eEt30= 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=UIIZ9gg9; 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="UIIZ9gg9" 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=bEerhIEewmfT/YDQQReJK1JTsyW1j7y6PqxaMsqRbsk=; b=UIIZ9gg9UVcpKnhAA2NPSRwT8t HELpMdpiEur3G0zUlIi8SbMec4UcZ78FqZqN/5aBiT36Djyj7Jpia0GCRo/oAwZQpbeYhlcfZ8O0M 2+jXSO2gHjawMEKrw/PFyVvgPY0G2Z57MUtVPNjPHDVg0JNWRZVsCNpSCP6c0LWNDsIhSoRog7i+K PzpgNC4WHY5zJadr/WUYHd918d2EVDJkETSx4SGQYnaKST5XEqiUZa0rdIQ1GQPGbjDrFE9l8vyaD MID0WWQODjz+0HvWee0uHjDo0s5tkC43X2eCddDN/ZLO82cqHyJJ/HTGRpvaNXljEBbLT8GPJBIPt sVmd68lQ==; 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 1wMUoV-00000008ZPr-3uOF; Mon, 11 May 2026 17:53:08 +0000 Message-ID: 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 , Marc Zyngier 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 18:53:06 +0100 In-Reply-To: References: <6856b269d2af706eae397e0cf9c1231f89d9a932.camel@infradead.org> <6afc4b95-3c15-4d71-877d-19b84e91ce05@redhat.com> <57bc082f4824d6114d3156744c25986effc29aca.camel@infradead.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-enZD6RcTNRAVwt9lhRh7" User-Agent: Evolution 3.52.3-0ubuntu1.1 Precedence: bulk X-Mailing-List: linux-doc@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 --=-enZD6RcTNRAVwt9lhRh7 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2026-05-11 at 18:56 +0200, Paolo Bonzini wrote: > On 5/11/26 18:38, David Woodhouse wrote: > > 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 = doesn'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. > >=20 > > 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. >=20 > Yep, these are the "quirks"---if it's too early for Arm to commit to=20 > that, I guess it's fine. >=20 > However, independent of this patch which I (obviously) believe is a good= =20 > idea, I'd like to understand how far it is, assuming 1) no quirks 2)=20 > same CPU host. It generally works out on arm64, although it's obviously a lot more work than x86 which makes an effort to get this stuff right. When we upgrade the kernel we do a lot of in-guest testing to find the stuff that=C2=A0"broke", like cache reporting: https://lore.kernel.org/all/254ca48a67779ccf9b9f60e2bb5796a305c03f95.camel@= infradead.org/ ... and the GICD_IIDR thing which I reposted today: https://lore.kernel.org/all/20260511113558.3325004-2-dwmw2@infradead.org/ Those are the ones I came up against recently because someone had just *reverted* the offending commits local in a previous kernel upgrade, and I'm trying to fix it *properly* this time around and not carry the reverts forward for ever. And fix the expectations too, of course. Being told that we shouldn't *expect* to be able to upgrade and roll back the kernel while remaining compatible is... not OK. > By the way, you didn't Cc Marc... Ah crap, I meant to. Thanks for spotting that! 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