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Mon, 10 May 2021 07:49:18 +0000 Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id E74B861432; Mon, 10 May 2021 07:49:15 +0000 (UTC) Received: from 78.163-31-62.static.virginmediabusiness.co.uk ([62.31.163.78] helo=why.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1lg0fB-000Jm2-TL; Mon, 10 May 2021 08:49:14 +0100 Date: Mon, 10 May 2021 08:49:13 +0100 Message-ID: <871rafowp2.wl-maz@kernel.org> From: Marc Zyngier To: Auger Eric Cc: Shaokun Zhang , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, linux-pci@vger.kernel.org, Alex Williamson , Cornelia Huck , Nianyao Tang , Bjorn Helgaas Subject: Re: Question on guest enable msi fail when using GICv4/4.1 In-Reply-To: References: <3a2c66d6-6ca0-8478-d24b-61e8e3241b20@hisilicon.com> <87k0oaq5jf.wl-maz@kernel.org> <878s4qq00u.wl-maz@kernel.org> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/27.1 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") X-SA-Exim-Connect-IP: 62.31.163.78 X-SA-Exim-Rcpt-To: eric.auger@redhat.com, zhangshaokun@hisilicon.com, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, linux-pci@vger.kernel.org, alex.williamson@redhat.com, cohuck@redhat.com, tangnianyao@huawei.com, bhelgaas@google.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210510_004916_651195_E1B79B03 X-CRM114-Status: GOOD ( 41.72 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi Eric, On Sun, 09 May 2021 18:00:04 +0100, Auger Eric wrote: > > Hi, > On 5/7/21 1:02 PM, Marc Zyngier wrote: > > On Fri, 07 May 2021 10:58:23 +0100, > > Shaokun Zhang wrote: > >> > >> Hi Marc, > >> > >> Thanks for your quick reply. > >> > >> On 2021/5/7 17:03, Marc Zyngier wrote: > >>> On Fri, 07 May 2021 06:57:04 +0100, > >>> Shaokun Zhang wrote: > >>>> > >>>> [This letter comes from Nianyao Tang] > >>>> > >>>> Hi, > >>>> > >>>> Using GICv4/4.1 and msi capability, guest vf driver requires 3 > >>>> vectors and enable msi, will lead to guest stuck. > >>> > >>> Stuck how? > >> > >> Guest serial does not response anymore and guest network shutdown. > >> > >>> > >>>> Qemu gets number of interrupts from Multiple Message Capable field > >>>> set by guest. This field is aligned to a power of 2(if a function > >>>> requires 3 vectors, it initializes it to 2). > >>> > >>> So I guess this is a MultiMSI device with 4 vectors, right? > >>> > >> > >> Yes, it can support maximum of 32 msi interrupts, and vf driver only use 3 msi. > >> > >>>> However, guest driver just sends 3 mapi-cmd to vits and 3 ite > >>>> entries is recorded in host. Vfio initializes msi interrupts using > >>>> the number of interrupts 4 provide by qemu. When it comes to the > >>>> 4th msi without ite in vits, in irq_bypass_register_producer, > >>>> producer and consumer will __connect fail, due to find_ite fail, and > >>>> do not resume guest. > >>> > >>> Let me rephrase this to check that I understand it: > >>> - The device has 4 vectors > >>> - The guest only create mappings for 3 of them > >>> - VFIO calls kvm_vgic_v4_set_forwarding() for each vector > >>> - KVM doesn't have a mapping for the 4th vector and returns an error > >>> - VFIO disable this 4th vector > >>> > >>> Is that correct? If yes, I don't understand why that impacts the guest > >>> at all. From what I can see, vfio_msi_set_vector_signal() just prints > >>> a message on the console and carries on. > >>> > >> > >> function calls: > >> --> vfio_msi_set_vector_signal > >> --> irq_bypass_register_producer > >> -->__connect > >> > >> in __connect, add_producer finally calls kvm_vgic_v4_set_forwarding > >> and fails to get the 4th mapping. When add_producer fail, it does > >> not call cons->start, calls kvm_arch_irq_bypass_start and then > >> kvm_arm_resume_guest. > > > > [+Eric, who wrote the irq_bypass infrastructure.] > > > > Ah, so the guest is actually paused, not in a livelock situation > > (which is how I interpreted "stuck"). > > > > I think we should handle this case gracefully, as there should be no > > expectation that the guest will be using this interrupt. Given that > > VFIO seems to be pretty unfazed when a producer fails, I'm temped to > > do the same thing and restart the guest. > > > > Also, __disconnect doesn't care about errors, so why should __connect > > have this odd behaviour? > > _disconnect() does not care as we should always succeed tearing off > things. del_* ops are void functions. On the opposite we can fail > setting up the bypass. > > Effectively > a979a6aa009f ("irqbypass: do not start cons/prod when failed connect") > needs to be reverted. > > I agree the kerneldoc comments in linux/irqbypass.h may be improved to > better explain the role of stop/start cbs and warn about their potential > global impact. Yup. It also begs the question of why we have producer callbacks, as nobody seems to use them. > wrt the case above, "in __connect, add_producer finally calls > kvm_vgic_v4_set_forwarding and fails to get the 4th mapping", shouldn't > we succeed in that case? >From a KVM perspective, we can't return a success because there is no guest LPI that matches the input signal. And such failure seems to be expected by the VFIO code, which just prints a message on the console and set the producer token to NULL. So returning an error from the KVM code is useful, at least to an extent. Thanks, M. -- Without deviation from the norm, progress is not possible. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel