Re: [Qemu-devel] [PATCH v3] intel_iommu: allow dynamic switch of IOMMU region

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 26 Dec 2016 10:45:55 +0800
Jason Wang <jasowang@redhat.com> wrote:

> On 2016年12月23日 11:26, Peter Xu wrote:
> > On Thu, Dec 22, 2016 at 07:34:10PM +0800, Jason Wang wrote:  
> >>
> >> On 2016年12月22日 19:04, Peter Xu wrote:  
> >>> On Thu, Dec 22, 2016 at 05:52:58PM +0800, Jason Wang wrote:  
> >>>> On 2016年12月22日 17:48, Peter Xu wrote:  
> >>>>>   /* Handle Translation Enable/Disable */
> >>>>>   static void vtd_handle_gcmd_te(IntelIOMMUState *s, bool en)
> >>>>>   {
> >>>>> +    if (s->dmar_enabled == en) {
> >>>>> +        return;
> >>>>> +    }
> >>>>> +
> >>>>>       VTD_DPRINTF(CSR, "Translation Enable %s", (en ? "on" : "off"));
> >>>>>       if (en) {
> >>>>> @@ -1196,6 +1237,8 @@ static void vtd_handle_gcmd_te(IntelIOMMUState *s, bool en)
> >>>>>           /* Ok - report back to driver */
> >>>>>           vtd_set_clear_mask_long(s, DMAR_GSTS_REG, VTD_GSTS_TES, 0);
> >>>>>       }
> >>>>> +
> >>>>> +    vtd_switch_address_space_all(s, en);
> >>>>>   }  
> >>>> We may need something like notifier here to tell e.g vhost to stop device
> >>>> IOTLB. (Since it's likely this series were applied after device IOTLB
> >>>> patches)  
> >>> Yes, I missed vhost case.
> >>>
> >>> To notify vhost, IMO we should be able to use memory listeners just
> >>> like how vfio devices do (please refer to vfio_memory_listener).  
> >> Just for switching? This seems an overkill since we don't depends on it for
> >> all other things. Guest should setup correct mappings by explicitly notify
> >> device IOTLB. A quick glance at ATS spec, for enabling and disabling, looks
> >> like it was done through enable bit of ASTctl instead of here.
> >>
> >> So we are probably ok here :)  
> > So we need a more general way to notify ATS about this, right? (e.g.,
> > for future devices that support ATS as well, not only vhost)  
> 
> Yes, if we want to make ATS visible to guest. But looks like it needs 
> more e.g VFIO support for device IOTLB invalidation.

ATS is already visible for vfio-pci devices, it will be enabled if the
host IOMMU supports ATS.  If the host IOMMU does not support ATS,
enabling it on the device by the guest will generate unsupported
requests.  This is yet another case, along with address width, where
it's potentially really complicated and not amenable to hotplug to have
a guest IOMMU.  AFAICT, there's nothing to be added to vfio for device
iotlb invalidations, this occurs automatically between the host IOMMU
and device when a DMA unmap occurs.  Thanks,

Alex