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 645E21EFFB7; Thu, 7 May 2026 06:58:18 +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=1778137104; cv=none; b=TIgk2uIFM+KswlmYTxNheYT5LP2sMXnZUTZ5+IOlp+EpR61y7yT5WuVBWUnj9dlcbNvyBD3PvXI/d1kcBJlJ108Vda5Xg/mthmHN2EWWQfbBc0EhosdDQRw6JSx0ge8tauFNkgqQG9Q6ZpbrzNCFFD5l4A/1wFkgWaVucvzRTjk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778137104; c=relaxed/simple; bh=mxjwDGPEe7OYJEMSjQmJ9dFEWud1XD6GErJf+tzhDOY=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=W22ZCos0FyvwWXq/qDw2lfozxKTaAZ43ei3v+Sq6PMP6jUuxz56OGECUhgFTBTVHGf5cpmyQ9EEJyCn/WWu9UBKbciVi2HxSTkDi6XwxySKKMtsGwXX0yKgG3mXF1E+NDFSY4o5xegF7FdsJ7rj6z4k/uAqoycuDf0+DG7Q7OlA= 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=DzdyBO1j; 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="DzdyBO1j" 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=GwYVxExkFstJ+bs4n64nO0YVdCLjI5WkC8emNVkbPS0=; b=DzdyBO1jk1wVt5M2JKsAZLNOnX eIx+GShD5ZFy1aE0CVV5G/KhHu6ljhDJB8lzPvOgNSHYEnTm4TmgCGl0wsL5H0bSPlskKHZwL6oNp 15hiN8xaYbKr6AcruP9pr8gdismZRBDOs2TcgurcFK/y7mnY8rcuu8fzmh+l2gtQkdNSHx1BSKUHq ZSdbmzWmx/pbu+zlMcZyxiizX6LzmXZ3JLpKA+wzHWBj/8K+7ICyv4uCGwBmVjkMsNrpJk6VOrjKE EXmsz/G5trM2qp7ladA/0zxTez8L450gs4Q0CppPYxEtd3NshC5nnNYydaXlOg/zWyNlScwDl+cPp gWEjmycw==; Received: from [172.31.31.148] (helo=u09cd745991455d.ant.amazon.com) by desiato.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wKsgK-00000002ksP-1ORk; Thu, 07 May 2026 06:58:01 +0000 Message-ID: <8e7bc66a7994ca06f164a5d5f7ceb3f07d3a1357.camel@infradead.org> Subject: Re: [PATCH] KVM: x86/xen: bail in IRQ context on PREEMPT_RT in kvm_xen_set_evtchn_fast() From: David Woodhouse To: Mauricio Faria de Oliveira , Paul Durrant , Sean Christopherson , Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt Cc: kernel-dev@igalia.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, syzbot+208f7f3e5f59c11aeb90@syzkaller.appspotmail.com Date: Thu, 07 May 2026 07:58:00 +0100 In-Reply-To: <20260506-xen-rt-sleep-v1-1-53b6b60a671d@igalia.com> References: <20260506-xen-rt-sleep-v1-1-53b6b60a671d@igalia.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-vWIr7WXR5ZJq3srgvHIf" 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 --=-vWIr7WXR5ZJq3srgvHIf Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, 2026-05-06 at 23:36 -0300, Mauricio Faria de Oliveira wrote: > kvm_xen_set_evtchn_fast() calls read_lock_irqsave(), which might block > on PREEMPT_RT, but that is invalid in IRQ context, as when it's called > by xen_timer_callback() (even on PREEMPT_RT per HRTIMER_MODE_ABS_HARD). >=20 > Check for that case, and bail out early. >=20 > Note: there is previous work and discussion on this [1] (~2 years ago), > which involved continuing to execute the function with changes, but it > was not merged. That was a different, more complex approach. >=20 > [1] https://lore.kernel.org/lkml/ZdPQVP7eejq3eFjc@google.com/ ...=20 > + /* Bail in IRQ context on PREEMPT_RT; read_lock_irqsave() might block *= / > + if (IS_ENABLED(CONFIG_PREEMPT_RT) && in_hardirq()) > + goto out; The approach in Paul's earlier patch was better; we absolutely *want* to deliver the interrupt to the guest immediately whenever we can, and only fall back to the workqueue in the rare case that the shared info page has been invalidated. We should switch to plain read_trylock(), *without* the local_irq_save(). And since this was the *only* case where the GPC lock was ever taken under IRQ=C2=B9, all the GPC locking can drop the _irq part. Sean's concern was: >> I am not comfortable applying this patch. As shown by the need for the = next patch >> to optimize unrelated invalidations, switching to read_trylock() is more= subtle >> than it seems at first glance. Specifically, there are no fairness guar= antees. I'm OK with that in this case. Because kvm_xen_set_evtchn_fast(), as with *everything* called from kvm_arch_set_irq_inatomic(), is explicitly designed to be a 'best effort' and allowed to return -EWOULDBLOCK when it's too hard. And the write lock being held here should a *rare* case, as the GPC for the shared_info and vcpu_info pages should basically *never* get invalidated while the guest is running. I've taken the same read_trylock() approach in=20 https://lore.kernel.org/all/1d6712ed413ea66ef376d1410811997c3b416e99.camel@= infradead.org/ --=-vWIr7WXR5ZJq3srgvHIf 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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