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 39B7130148A; Tue, 2 Jun 2026 12:37:17 +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=1780403839; cv=none; b=aEUqO8VMIHlrJAlqXdpiC0PfUotlXT9i+3QG9LwzH/w3YI1ANSo/mNSiUHfiHQkgsLOxM+62ZERgCpvlRTyMyjY45mOfeCjMkIcqmVBkwlnzqr+vsv7aKqb47xOPc8VT48A0aDWDWrTonRNJsDQNOGCRxufDo1rlPFYGDa1J+uY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780403839; c=relaxed/simple; bh=OCCLL5SwGu4xH+cgJtdC4qx2BKJXREzwT7+DWS3BLFU=; h=Message-ID:Subject:From:To:Cc:Date:in-reply-to:Content-Type: MIME-Version; b=GS0vLJ/YAHtDtlrd1c47Y8FTMlKW+9EgeDc0A7o0E/KcxsSYWNQynRGi2WXSYV7tpKIiIbKIGk701xiEoSc9/Yowb9Nd/Ui+1R0ySp8ngQ7o9qYTWndO1HPr/stUK2tRCMpmH82UuR7ifSTIq33raVKGgy0nzpN6d+A3KXjGXwA= 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=kl6CoK2i; 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="kl6CoK2i" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=MIME-Version:Content-Type:in-reply-to: Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To:Content-Transfer-Encoding: Content-ID:Content-Description:References; bh=Rb5yfIwgrBRrR87AJg7XBlvLHfCDmy+gpwdr9xfXtVA=; b=kl6CoK2inRLDsF2+v5xJ4s/imP 211nfAkrvv6yzbQvGzTBOUPqIUlHS+BlLPJhwDz7PA14FzglYo0bkMPBUoBtmQbc7PiWF30UXV/hD tVLeFBnMcAuqZdbIuHuXa5kFBpQCqplVjb3IcugmV9mV9tNJyvyJ5YyK/VSPy3nlgyaqHpVNHl1AP 2AL20Jsvkr93JLvO4WflzF07b9EhsVTBauPLJ/xSgFpz6YYZ0LACuT4LiewWTGfUYFOFgC6BzVnKw ni0U6VgWny5yUkHFh9i+ZiW2fY0Ki4+se+RPfimfu9AnqBptC36xHiVRe/PGVvWn0amCA0Q6t9+8g hbB8E7rQ==; Received: from [2001:8b0:10b:5:5b40:b57d:669c:36f4] (helo=u09cd745991455d.ant.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wUOMo-00000001wQi-3KfG; Tue, 02 Jun 2026 12:37:11 +0000 Message-ID: Subject: Re: [PATCH v2 15/20] KVM: x86/xen: Convert kvm_xen_set_evtchn_fast() to gpc's CLASS() APIs From: David Woodhouse To: seanjc@google.com Cc: pbonzini@redhat.com, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, sashiko-reviews@lists.linux.dev, dwmw@amazon.co.uk Date: Tue, 02 Jun 2026 13:37:10 +0100 in-reply-to: Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-J5W3Jn6TTVM29ghdH7g3" 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 --=-J5W3Jn6TTVM29ghdH7g3 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 29 May 2026 12:11:37 -0700, Sean Christopherson wrote: > On Fri, May 29, 2026, sashiko-bot@kernel.org wrote: > > [Severity: High] > > Does this unintentionally hold the gpc lock across the IPI kick? >=20 > No, it intentionally holds the gpc lock across _sending_ the IPI kick. >=20 > > [Severity: High] > > Does this now hold the VM-wide shinfo_cache lock while calling > > __kvm_xen_set_evtchn_fast() and kicking the vCPU? > >=20 > > Since shinfo_map is a function-scoped CLASS() variable, its destructor > > won't release the lock until after __kvm_xen_set_evtchn_fast() returns. > > This creates a nested locking dependency and holds locks over expensive > > cross-vCPU operations, potentially serializing event channel deliveries > > across the entire VM on the fast path. >=20 > __kvm_vcpu_kick() is neither expensive nor cross-vCPU. In the wait=3Dfal= se case, > which is the behavior of kvm_vcpu_kick(), it sends IPIs via smp_send_resc= hedule(), > i.e. it's more or less just __apic_send_IPI(cpu, RESCHEDULE_VECTOR), whic= h is a > single WRMSR on modern harware. Hm, if we really are going to use a raw rwlock then I suppose we should take care not to spend any extra cycles with the lock held unless we actually *need* to? In which case I wonder if we really need the scoped CLASS() thing (that encourages people to run to the end of the function under the lock), or if the helpers you add for that are sufficient on their own? --=-J5W3Jn6TTVM29ghdH7g3 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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