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

[Duan, Zhenzhong]

>-----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.

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