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Subject: Re: [virtio-dev] Timing out virtio-pci config space access
On Fri, Nov 05, 2021 at 05:59:43PM +0530, Srivatsa Vaddagiri wrote: > * Michael S. Tsirkin <mst@redhat.com> [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
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