Re: [Qemu-devel] [PATCH RFC v3 01/14] IOMMU: add option to enable VTD_CAP_CM to vIOMMU capility exposoed to guest

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Peter Xu [mailto:peterx@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