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

[Jason Wang]

On Fri, Jan 24, 2025 at 12:01 PM Duan, Zhenzhong
<zhenzhong.duan@intel.com> wrote:
>
>
>
> >-----Original Message-----
> >From: Jason Wang <jasowang@redhat.com>
> >Subject: Re: [PATCH] hw/virtio/vhost: Disable IOTLB callbacks when IOMMU gets
> >disabled
> >
> >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.
>
> Oh, I see.
>
> >>
> >> 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.
>
> Have not access😊

It should work now.

Thanks

>
> >
> >Thanks
> >
> >>
> >> A quick try might be to provide a .driver.shutdown to
> >> virtio_net_driver structure and reset the device there as a start.
>
> Make sense.
>
> Thanks
> Zhenzhong