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 A571A36D503; Thu, 7 May 2026 15:22:41 +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=1778167365; cv=none; b=fcwZlEY3i5zJFo/d+NZxnJK4uKbYl5yhAhwtdpRam3jkHkFnmHDOUCD2ueCzN+0v+7FglAsHsZaFV2jf90Suo63SkLCiEZqqyPM939AUD8U7j9x25zSBCUC1h09PY3Z+ohlZPYLwJ05vp9fkY1jVJWD0XfbAV6qXvGIB1XtNqYI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778167365; c=relaxed/simple; bh=gQCIv+0Rk7QaQ7hoLtSc8KzXHRcd3oSTSkIT6Q1T3BU=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=nJv6MaYzG9KmV87UN1dpGcZo1L28vJm3MF3gKV8jrWXoCZ+PhpCFt3coGPmtBk4ZVE47eeE61pjQgYDUbEg5I2uPRw+fPxYTvaOkI4/phjT8/jDl7lUn4jMLi2t7VTfa67z0Dnz/vKMkRwiUMKYTiCso35X5WsmgZnR2zlDSXxc= 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=qFcNXio4; 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="qFcNXio4" 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=QTucYTxb3ZEm/mr/ifQJhhxEirQO/2Pe1y7GAASxNd0=; b=qFcNXio4mtvPKYu2QvsR6x09GX ooekun8CWk2FOi/5EwyDSk6oh+wDzgQPr3ZCUWx5FZoy58iu2vgl4FvCuMxVyvu2ctpGxgwLwRPDi /3cDFDu0omwrovH2NnD2kxL0IhM74urir6Zg3wKvCAou4uA3bhOx0fpbEkpIU9fso1njRlzPv4ZdF BVyVbkcNkLDkRzgqQY4MJ+oYd25mrapEhN/7YmNPThNdPv8Ye6pVTsyR/TEU1DlZp3YyvJuXvchx3 0xmakSwBcEQz/Y/jeIyB4ARoBTGsiZM5DuV75W50KjbPntBR/ffjSDEilvopopHGxU6fku0Wi6+pG a2fx1Kww==; 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 1wL0YU-00000003toK-3Spk; Thu, 07 May 2026 15:22:27 +0000 Message-ID: <876c720ab963f550d3166b067f6457fcd78ec415.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 Cc: 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 , 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 16:22:26 +0100 In-Reply-To: References: <20260506-xen-rt-sleep-v1-1-53b6b60a671d@igalia.com> <8e7bc66a7994ca06f164a5d5f7ceb3f07d3a1357.camel@infradead.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-KAfkmaggwtdyfy2U8XuO" 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 --=-KAfkmaggwtdyfy2U8XuO Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2026-05-07 at 11:56 -0300, Mauricio Faria de Oliveira wrote: > On 2026-05-07 03:58, David Woodhouse wrote: > > 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 bloc= k > > > on PREEMPT_RT, but that is invalid in IRQ context, as when it's calle= d > > > 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 i= t > > > was not merged. That was a different, more complex approach. > > >=20 > > > [1] https://lore.kernel.org/lkml/ZdPQVP7eejq3eFjc@google.com/ > >=20 > > ... > >=20 > > > + /* Bail in IRQ context on PREEMPT_RT; read_lock_irqsave() might blo= ck */ > > > + if (IS_ENABLED(CONFIG_PREEMPT_RT) && in_hardirq()) > > > + goto out; > >=20 > >=20 > > 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. >=20 > Certainly, that was better. This was a simple workaround, but with this > clarification, it indeed doesn't fit. >=20 > > 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 p= art. >=20 > Ok, I can take a look. Or do you plan to work on it yourself (as you > hit the issue with read_unlock later in this thread)? I'm working on it now; thanks. Currently confused by the fact that the read_trylock() seems to fail more often than it should under RT, causing the fallback path to be taken... and the fallback path doesn't seem to work properly... Your version should have seen this too, surely? Did the selftest in linux/tools/testing/selftests/kvm/x86/xen_shinfo_test work with your patch? --=-KAfkmaggwtdyfy2U8XuO 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 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