> From: Peter Xu [mailto:pet...@redhat.com] > Sent: Friday, January 20, 2017 5:12 PM > > On Fri, Jan 20, 2017 at 08:59:01AM +0000, Tian, Kevin wrote: > > [...] > > > > > Also for hot-add > > > > device path, some check of caching mode is required. If not set, > > > > should we fail hot-add operation? I don't think we have such physical > > > > platform with some devices behind IOMMU while others not. > > > > > > Could you explain in what case will we fail a hot plug? > > > > > > > user enables intel-iommu, but don't set caching mode. > > > > Then later user hot-add a PCI device to the VM. Guest will assume > > newly assigned device also behind the default vIOMMU, and thus > > needs to setup IOVA mappings, which is then broken... > > Is the newly added device a vfio-pci device? If so, we should hit > this and VM will stops to work: > > if (!s->cache_mode_enabled && new & IOMMU_NOTIFIER_MAP) { > error_report("We need to set cache_mode=1 for intel-iommu to enable " > "device assignment with IOMMU protection."); > exit(1); > }
sorry I didn't found this code. In which code path is it hit? > > I admit this is not user-friendly, and a better way may be that we > disallow the hot-plug in that case, telling the user about the error, > rather than crashing the VM. But, I think that can be a patch outside > this series, considering (again) that this only affects advanced > users. > Crashing VM is bad.... but anyway, I'll leave maintainer to decide whether they'd like it fixed now or later. :-) Thanks Kevin
