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 5F7242877D4; Tue, 27 Jan 2026 22:37:17 +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=1769553440; cv=none; b=L3eJvl+h81LBsGquq1VRMcIbWdgAYZd4hb9xV6c6sRiTWuhhZin3roiuZ++pqL52nq82HFhEx4OhZjWNgSrdSGb5XBJyGWpCCfnTtgcKOMR6/heQhJX55mBWFqPD2tbCyGjUVKqoq6H7gAZyB4Koj7ceOBm1W3d5b2krbsN+9SY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769553440; c=relaxed/simple; bh=hRYfwU7cq1nIPMH2K1WOzIxEoRfE95X12OZy5NPVje8=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=csGIl+3OUSOCPdiR6tu/4TDAnSJbKNkx3VqGVgY8lN6bnokBzEDeWuob0pQmLed+/TUPaSQOlKm3QGE0yP33bZhqW5JlZt2anfHJwIYcpEgHRmRStlKZQ5VcN6nqcWgx5CPxFDHAmDQmEXZNE5fBYzX/A+5hCa9uWeOc2mwTTqQ= 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=WOcG634d; 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="WOcG634d" 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=M+TIob0kvqofpEUCljjJiI1nIcKGQracjWOpYbBz3Os=; b=WOcG634dyqANK3k1noHl9wXF7i Hkt8obvRVq7mY6mzlU6PDRSo6lYoL9ITX5ZgVOrkgSel5JS71UtNh6+ReV3FD7EILjj/8Sx6CUp2D ayzxt3WFCEgHlcoueBIXklOdpUyTVKZIjbIcgWX5yF7awjsR/Dh+WqcIuAEuVfPSyp4lEsCo5+P40 AwCRgIlGAqRGIp/9f85zdehYGTWZ4paSOV+2Q1mNKW/WaX4TuREx/J7qLSaC4x4bDkX6LZdJ7GTH4 WPJLXR1N/4cpFf8UCpwXIGzUTxjnV+W3tdFIHUzR4PegPcL5+3ymG9bWGKeSoNWjsee5AXUrq8Dr3 eU1qv4aA==; Received: from [2001:8b0:10b::4] (helo=u09cd745991455d.ant.amazon.com) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vkrgI-00000007hHl-07tI; Tue, 27 Jan 2026 22:37:06 +0000 Message-ID: Subject: Re: [PATCH v6] KVM: x86: Add x2APIC "features" to control EOI broadcast suppression From: David Woodhouse To: Sean Christopherson Cc: Khushit Shah , pbonzini@redhat.com, kai.huang@intel.com, mingo@redhat.com, x86@kernel.org, bp@alien8.de, hpa@zytor.com, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, dave.hansen@linux.intel.com, tglx@linutronix.de, jon@nutanix.com, shaju.abraham@nutanix.com, stable@vger.kernel.org Date: Tue, 27 Jan 2026 14:36:57 -0800 In-Reply-To: References: <20260123125657.3384063-1-khushit.shah@nutanix.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-P1PfO+yc48Cghxnt4i+4" 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 --=-P1PfO+yc48Cghxnt4i+4 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue, 2026-01-27 at 13:49 -0800, Sean Christopherson wrote: >=20 > Nope, we should be good on that front, kvm->arch.irqchip_mode can't be ch= anged > once its set.=C2=A0 I.e. the irqchip_split() check could get a false nega= tive if it's > racing with KVM_CREATE_IRQCHIP, but it can't get a false positive and thu= s > incorrectly allow KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST. Ah, so userspace which checks all the kernel's capabilities *first* will not see KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST advertised, because it needs to enable KVM_CAP_SPLIT_IRQCHIP first? I guess that's tolerable=C2=B9 but the documentation could make it clearer, perhaps? I can see VMMs silently failing to detect the feature because they just don't set split-irqchip before checking for it?=20 =C2=B9 although I still kind of hate it and would have preferred to have th= e I/O APIC patch; userspace still has to intentionally *enable* that combination. But OK, I've reluctantly conceded that. --=-P1PfO+yc48Cghxnt4i+4 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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