Re: [PATCH] hw/virtio/vhost: Disable IOTLB callbacks when IOMMU gets disabled

[Jason Wang]

On Fri, Jan 24, 2025 at 11:30 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Fri, Jan 24, 2025 at 10:44 AM Duan, Zhenzhong
> <zhenzhong.duan@intel.com> wrote:
> >
> >
> >
> > >-----Original Message-----
> > >From: Eric Auger <eric.auger@redhat.com>
> > >Subject: Re: [PATCH] hw/virtio/vhost: Disable IOTLB callbacks when IOMMU 
> > >gets
> > >disabled
> > >
> > >Hi Jason,
> > >
> > >
> > >On 1/23/25 2:34 AM, Jason Wang wrote:
> > >> On Wed, Jan 22, 2025 at 3:55 PM Eric Auger <eric.auger@redhat.com> wrote:
> > >>> Hi Jason,
> > >>>
> > >>>
> > >>> On 1/22/25 8:17 AM, Jason Wang wrote:
> > >>>> On Wed, Jan 22, 2025 at 12:25 AM Eric Auger <eric.auger@redhat.com>
> > >wrote:
> > >>>>> Hi Jason,
> > >>>>>
> > >>>>> On 1/21/25 4:27 AM, Jason Wang wrote:
> > >>>>>> On Tue, Jan 21, 2025 at 1:33 AM Eric Auger <eric.auger@redhat.com>
> > >wrote:
> > >>>>>>> When a guest exposed with a vhost device and protected by an
> > >>>>>>> intel IOMMU gets rebooted, we sometimes observe a spurious warning:
> > >>>>>>>
> > >>>>>>> Fail to lookup the translated address ffffe000
> > >>>>>>>
> > >>>>>>> We observe that the IOMMU gets disabled through a write to the 
> > >>>>>>> global
> > >>>>>>> command register (CMAR_GCMD.TE) before the vhost device gets
> > >stopped.
> > >>>>>>> When this warning happens it can be observed an inflight IOTLB
> > >>>>>>> miss occurs after the IOMMU disable and before the vhost stop. In
> > >>>>>>> that case a flat translation occurs and the check in
> > >>>>>>> vhost_memory_region_lookup() fails.
> > >>>>>>>
> > >>>>>>> Let's disable the IOTLB callbacks when all IOMMU MRs have been
> > >>>>>>> unregistered.
> > >>>>>>>
> > >>>>>>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> > >>>>>>> ---
> > >>>>>>>  hw/virtio/vhost.c | 4 ++++
> > >>>>>>>  1 file changed, 4 insertions(+)
> > >>>>>>>
> > >>>>>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > >>>>>>> index 6aa72fd434..128c2ab094 100644
> > >>>>>>> --- a/hw/virtio/vhost.c
> > >>>>>>> +++ b/hw/virtio/vhost.c
> > >>>>>>> @@ -931,6 +931,10 @@ static void
> > >vhost_iommu_region_del(MemoryListener *listener,
> > >>>>>>>              break;
> > >>>>>>>          }
> > >>>>>>>      }
> > >>>>>>> +    if (QLIST_EMPTY(&dev->iommu_list) &&
> > >>>>>>> +        dev->vhost_ops->vhost_set_iotlb_callback) {
> > >>>>>>> +        dev->vhost_ops->vhost_set_iotlb_callback(dev, false);
> > >>>>>>> +    }
> > >>>>>> So the current code assumes:
> > >>>>>>
> > >>>>>> 1) IOMMU is enabled before vhost starts
> > >>>>>> 2) IOMMU is disabled after vhost stops
> > >>>>>>
> > >>>>>> This patch seems to fix 2) but not 1). Do we need to deal with the
> > >>>>>> IOMMU enabled after vhost starts?
> > >>>>> sorry I initially misunderstood the above comment. Indeed in the 
> > >>>>> reboot
> > >>>>> case assumption 2) happens to be wrong. However what I currently do 
> > >>>>> is:
> > >>>>> stop listening to iotlb miss requests from the kernel because my
> > >>>>> understanding is those requests are just spurious ones, generate
> > >>>>> warnings and we do not care since we are rebooting the system.
> > >>>>>
> > >>>>> However I do not claim this could handle the case where the IOMMU MR
> > >>>>> would be turned off and then turned on. I think in that case we should
> > >>>>> also flush the kernel IOTLB and this is not taken care of in this 
> > >>>>> patch.
> > >>>>> Is it a relevant use case?
> > >>>> Not sure.
> > >>>>
> > >>>>> wrt removing assumption 1) and allow IOMMU enabled after vhost start. 
> > >>>>> Is
> > >>>>> that a valid use case as the virtio driver is using the dma api?
> > >>>> It should not be but we can't assume the behaviour of the guest. It
> > >>>> could be buggy or even malicious.
> > >>> agreed
> > >>>> Btw, we had the following codes while handling te:
> > >>>>
> > >>>> /* Handle Translation Enable/Disable */
> > >>>> static void vtd_handle_gcmd_te(IntelIOMMUState *s, bool en)
> > >>>> {
> > >>>>     if (s->dmar_enabled == en) {
> > >>>>         return;
> > >>>>     }
> > >>>>
> > >>>>     trace_vtd_dmar_enable(en);
> > >>>>
> > >>>> ...
> > >>>>
> > >>>>     vtd_reset_caches(s);
> > >>>>     vtd_address_space_refresh_all(s);
> > >>>> }
> > >>>>
> > >>>> vtd_address_space_refresh_all() will basically disable the iommu
> > >>>> memory region. It looks not sufficient to trigger the region_del
> > >>>> callback, maybe we should delete the region or introduce listener
> > >>>> callback?
> > >>> This is exactly the code path which is entered in my use case.
> > >>>
> > >>> vtd_address_space_refresh_all(s) induces the vhost_iommu_region_del. But
> > >given the current implement of this latter the IOTLB callback is not unset 
> > >and the
> > >kernel IOTLB is not refreshed. Also as I pointed out the  
> > >hdev->mem->regions are
> > >not updated? shouldn't they. Can you explain what they correspond to?
> > >> Adding Peter for more ideas.
> > >>
> > >> I think it's better to find a way to trigger the listener here, probably:
> > >>
> > >> 1) add/delete the memory regions instead of enable/disable
> > >sorry I don't understand what you mean. The vhost_iommu_region_del call
> > >stack is provided below [1]. Write to the intel iommu GCMD TE bit
> > >induces a call to vhost_iommu_region_del. This happens before the
> > >vhost_dev_stop whose call stack is provided below [2] and originates
> > >from a bus reset.
> > >
> > >We may have inflight IOTLB miss requests happening between both.
> > >
> > >If this happens, vhost_device_iotlb_miss() fails because the IOVA is not
> > >translated anymore by the IOMMU and the iotlb.translated_addr returned
> > >by address_space_get_iotlb_entry() is not within the registered
> > >vhost_memory_regions looked up in vhost_memory_region_lookup(), hence
> > >the "Fail to lookup the translated address" message.
> > >
> > >It sounds wrong that vhost keeps on using IOVAs that are not translated
> > >anymore. It looks we have a reset ordering issue and this patch is just
> > >removing the sympton and not the cause.
> > >
> > >At the moment I don't really get what is initiating the intel iommu TE
> > >bit write. This is a guest action but is it initiated from a misordered
> > >qemu event?
> >
> > During reboot, native_machine_shutdown() calls x86_platform.iommu_shutdown()
> > to disable iommu before reset. So Peter's patch will not address your issue.
> >
> > Before iommu shutdown, device_shutdown() is called to shutdown all devices.
> > Not clear why vhost is still active.
>
> It might be because neither virtio bus nor virtio-net provides a
> shutdown method.
>
> There used to be requests to provide those to unbreak the kexec.

More could be seen at https://issues.redhat.com/browse/RHEL-331

This looks exactly the same issue.

Thanks

>
> A quick try might be to provide a .driver.shutdown to
> virtio_net_driver structure and reset the device there as a start.
>
> Thanks
>
> >
> > Thanks
> > Zhenzhong
> >
> > >
> > >It could also relate to
> > >[PATCH 0/4] intel_iommu: Reset vIOMMU after all the rest of devices
> > >https://lore.kernel.org/all/?q=s%3Aintel_iommu%3A+Reset+vIOMMU+after+all+
> > >the+rest+of+devices
> >