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 A11EB2DEA9D; Thu, 7 May 2026 07:30:47 +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=1778139052; cv=none; b=ArjSmzIHNciMK5g/IuUfLCezJ1q5nmojVNsm8nDYcEs4Y4S9t3xPYrTzFkssmLSsNSjis15uC2cb6PGB4Pw9SXCTRLT6MKwNNIBF97T7OhfNNE23SPwZJtnibRqV6SUDzNzzQkNYaP6JFyf9Cs297yjEw3sMpM92uzdh5njIlSA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778139052; c=relaxed/simple; bh=RoDy7UuuGdgIo0U1zmR8tFTojAiGxY5dp7kwtzEzWh4=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=cCzTB1qThuRb34UhnapIPrwf2sWcLCX4ZbWk+Tzy9D13UCvqtWqYyiNwhWMJasqqrHl1xA5yeLRPFBkXk9T9kVsAxP+Ozj+/8MbGaEejOaXvS6mWcYJswMJYBcPVmILSy1RFtCSdfzhMsjLvcj4cys5RurGwyWmDFTWnskToYbU= 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=TnlvZ52m; 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="TnlvZ52m" 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=RoDy7UuuGdgIo0U1zmR8tFTojAiGxY5dp7kwtzEzWh4=; b=TnlvZ52mhHMQQpx/VXfrVY1Vhp QZ03yIDfUTFufg6xTExSKrHUhWY75ZrWDdd1u1bj3Do8hbLqeVjVe7lHiuboN9kV+Cx8xjCKDX7Fu 1w7aKtPL8CGBIDqFwt9CdlvGE5pWaehzBTFgFgurMNiePUQwLGV8T0Dumcpi9Fc/BD8nPG2VXeyeh UdBwfyRwfpEcY0GKYbH6HQEfvRbE8CKyurTZnrm/H23NU2CiVpc2DdsnhvVmlTElZwXhkswpneA3M MeDSX336BfnsrPzQ/lWIXejcfHjL+5qHfJEwQDa2cCajjgAcc/Cg7fH3/kY3vItrvJepQ0kS3Sz/t cqC19q0w==; 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 1wKtBl-00000002o7i-2Gv8; Thu, 07 May 2026 07:30:32 +0000 Message-ID: <3ba6984cfd94aea7202d463e98fe71e5032eafe1.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: Sebastian Andrzej Siewior Cc: Mauricio Faria de Oliveira , Paul Durrant , Sean Christopherson , Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , 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 08:30:29 +0100 In-Reply-To: <20260507071235.rdb4aGeE@linutronix.de> References: <20260506-xen-rt-sleep-v1-1-53b6b60a671d@igalia.com> <8e7bc66a7994ca06f164a5d5f7ceb3f07d3a1357.camel@infradead.org> <20260507071235.rdb4aGeE@linutronix.de> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-xdt1/KyyEjS1YyN1SOwp" 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 --=-xdt1/KyyEjS1YyN1SOwp Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2026-05-07 at 09:12 +0200, Sebastian Andrzej Siewior wrote: > On 2026-05-07 07:58:00 [+0100], David Woodhouse wrote: > > > > I am not comfortable applying this patch.=C2=A0 As shown by the nee= d for the next patch > > > > to optimize unrelated invalidations, switching to read_trylock() is= more subtle > > > > than it seems at first glance.=C2=A0 Specifically, there are no fai= rness guarantees. > >=20 > > 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. > >=20 > > 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. > >=20 > > I've taken the same read_trylock() approach in=20 > > https://lore.kernel.org/all/1d6712ed413ea66ef376d1410811997c3b416e99.ca= mel@infradead.org/ >=20 > So the cited patch does not look bad. That read-trylock should be fine > on RT (as in I don't see anything wrong with it). What did happen to it? Nothing, but it's addressing a different problem, not the kvm_xen_set_evtchn_fast() problem. --=-xdt1/KyyEjS1YyN1SOwp 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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