From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id 4D152986453 for ; Fri, 5 Nov 2021 13:13:35 +0000 (UTC) Date: Fri, 5 Nov 2021 09:13:27 -0400 From: "Michael S. Tsirkin" Message-ID: <20211105091234-mutt-send-email-mst@kernel.org> References: <20211104170740.GA14929@quicinc.com> <20211105033354-mutt-send-email-mst@kernel.org> <20211105122943.GA18377@quicinc.com> MIME-Version: 1.0 In-Reply-To: <20211105122943.GA18377@quicinc.com> Subject: Re: [virtio-dev] Timing out virtio-pci config space access Content-Type: text/plain; charset=us-ascii Content-Disposition: inline To: Srivatsa Vaddagiri Cc: virtio-dev@lists.oasis-open.org, jasowang@redhat.com List-ID: On Fri, Nov 05, 2021 at 05:59:43PM +0530, Srivatsa Vaddagiri wrote: > * Michael S. Tsirkin [2021-11-05 03:38:39]: > > > On Thu, Nov 04, 2021 at 10:37:40PM +0530, Srivatsa Vaddagiri wrote: > > > We are working on a virtio-pci implementation on a Type-1 hypervisor where > > > backend drivers are hosted in another VM and are considered untrusted. PCI is > > > the virtio transport used in this case. > > > > > > One issue that crops up is a read/write of config space can potentially block > > > forever, as the backend is untrusted and could be causing a denial-of-service of > > > sorts. This causes the vcpu to stall forever. I was wondering if we can timeout > > > in such case and have the hypervisor break the stall by letting read return > > > "error" (-1) along with setting DEVICE_NEEDS_RESET in status register. Will that > > > allow Linux guest driver to gracefully fail its probe? I don't see where Linux > > > handles DEVICE_NEEDS_RESET currently and also am not sure if returning -1 will > > > lead to graceful failure of the driver alone (we don't want VM to come down or > > > panic because of a mis-behaving device). > > > > DEVICE_NEEDS_RESET isn't handled ATM. the point of it in any case > > is a recoverable error, with a malicious backend this is > > not the case. > > > > > > Once thing you can do that will work a bit better is implementing > > surprise-removal in this case. > > My layman understanding of surprise removal is that it requires the PCI > controller to interrupt OS and convey which device is removed, so that the PCI > subsystem can mark it "removed"? Is that possible for the generic controller > ("pci-host-ecam-generic") that virtio pci devices use? I think so, yes. > > So hypervisor detects a timeout > > (presumably it knows what to expect of the device) and then pretends to > > guest device is gone, unmapping it completely from guest. > > Can you elaborate on what unmapping means? I think the reads should > return -1 and writes to be dropped in such case - beyond that what would unmap > entail? > > Thanks > vatsa Removing guest access to device so access attempts end up in QEMU. -- MST --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org