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

[Duan, Zhenzhong]

>-----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😊

>
>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