On 5/28/20 1:21 PM, Cornelia Huck wrote: > On Fri, 22 May 2020 23:04:51 +0200 > Halil Pasic <pa...@linux.ibm.com> wrote: > >> On Wed, 20 May 2020 12:23:24 -0400 >> "Michael S. Tsirkin" <m...@redhat.com> wrote: >> >>> On Fri, May 15, 2020 at 12:11:55AM +0200, Halil Pasic wrote: >>>> The virtio specification tells that the device is to present >>>> VIRTIO_F_ACCESS_PLATFORM (a.k.a. VIRTIO_F_IOMMU_PLATFORM) when the >>>> device "can only access certain memory addresses with said access >>>> specified and/or granted by the platform". This is the case for a >>>> protected VMs, as the device can access only memory addresses that are >>>> in pages that are currently shared (only the guest can share/unsare its >>>> pages). >>>> >>>> No VM, however, starts out as a protected VM, but some VMs may be >>>> converted to protected VMs if the guest decides so. >>>> >>>> Making the end user explicitly manage the VIRTIO_F_ACCESS_PLATFORM via >>>> the property iommu_on is a minor disaster. Since the correctness of the >>>> paravirtualized virtio devices depends (and thus in a sense the >>>> correctness of the hypervisor) it, then the hypervisor should have the >>>> last word about whether VIRTIO_F_ACCESS_PLATFORM is to be presented or >>>> not. >>> >>> So, how about this: switch iommu to on/off/auto. >> >> Many thanks for the reveiw, and sorry about the delay on my side. We >> have holidays here in Germany and I was not motivated enough up until >> now to check on my mails. >> >> >> I've actually played with the thought of switching iommu_platform to >> 'on/off/auto', but I didn't find an easy way to do it. I will look >> again. This would be the first property of this kind in QEMU, or? > > virtio-pci uses it for 'disable-legacy'. > >> >> The 'on/off/auto' would be certainly much cleaner form user-interface >> perspective. The downsides are that it is more invasive, and more >> complicated. I'm afraid that it would also leave more possibilities for >> user error. > > To me, on/off/auto sounds like a reasonable thing to do. > > What possibilities of 'user error' do you see? Shouldn't we fence off > misconfigurations, if the consequences would be disastrous? > >> >>> Add a property with a >>> reasonable name "allow protected"? If set allow switch to protected >>> memory and also set iommu auto to on by default. If not set then don't. >>> >> >> I think we have "allow protected" already expressed via cpu models. I'm >> also not sure how libvirt would react to the idea of a new machine >> property for this. You did mean "allow protected" as machine property, >> or? > > "Unpack facility in cpu model" means "guest may transition into pv > mode", right? What does it look like when the guest actually has > transitioned?
Well, we don't sync the features that the protected guest has back into QEMU. So basically the VM doesn't really change except for ms->pv now being true. > >> >> AFAIU "allow protected" would be required for the !PV to PV switch, and >> we would have to reject paravirtualized devices with iommu_platform='off' >> on VM construction or hotplug (iommu_platform='auto/on' would be fine). >> >> Could you please confirm that I understood this correctly? >> >> >>> This will come handy for other things like migrating to hosts without >>> protected memory support. >>> >> >> This is already covered by cpu model AFAIK. > > I don't think we'd want to migrate between pv and non-pv anyway? What exactly do you mean by that? I'd expect that the VM can either be migrated in PV or non-PV mode and not in a transition phase. > >> >>> >>> Also, virtio now calls this PLATFORM_ACCESS, maybe we should rename >>> the property (keeping old one around for compat)? >> >> You mean the like rename 'iommu_platform' to 'platform_access'? I like >> the idea, but I'm not sure libvirt will like it as well. Boris any >> opinions? >> >>> I feel this will address lots of complaints ... >>> >>>> Currently presenting a PV guest with a (paravirtualized) virtio-ccw >>>> device has catastrophic consequences for the VM (after the hypervisors >>>> access to protected memory). This is especially grave in case of device >>>> hotplug (because in this case the guest is more likely to be in the >>>> middle of something important). >>>> >>>> Let us manage the VIRTIO_F_ACCESS_PLATFORM virtio feature automatically >>>> for virtio-ccw devices, i.e. force it before we start the protected VM. >>>> If the VM should cease to be protected, the original value is restored. >>>> >>>> Signed-off-by: Halil Pasic <pa...@linux.ibm.com> >>> >>> >>> I don't really understand things fully but it looks like you are >>> changing features of a device. If so this bothers me, resets >>> happen at random times while driver is active, and we never >>> expect features to change. >>> >> >> Changing the device features is IMHO all right because the features can >> change only immediately after a system reset and before the first vCPU >> is run. That is ensured by two facts. >> >> >> First, the feature can only change when ms->pv changes. That is on the >> first reset after the VM entered or left the "protected virtualization" >> mode of operation. And that switch requires a system reset. Because the >> PV switch is initiated by the guest, and the guest is rebooted as a >> consequence, the guest will never observe the change in features. > > This really needs more comments, as it is not obvious to the casual > reader. (I also stumbled over the resets.) > > But I wonder whether we are actually missing those subsystems resets > today? > >
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