Re: [RFC] vhost: Clear the pending messages on vhost_init_device_iotlb()

[Eric Auger <eric.auger@redhat.com>]

On 11/8/22 04:09, Jason Wang wrote:
> On Tue, Nov 8, 2022 at 7:06 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>> On Mon, Nov 07, 2022 at 10:10:06PM +0100, Eric Auger wrote:
>>> Hi Michael,
>>> On 11/7/22 21:42, Michael S. Tsirkin wrote:
>>>> On Mon, Nov 07, 2022 at 09:34:31PM +0100, Eric Auger wrote:
>>>>> When the vhost iotlb is used along with a guest virtual iommu
>>>>> and the guest gets rebooted, some MISS messages may have been
>>>>> recorded just before the reboot and spuriously executed by
>>>>> the virtual iommu after the reboot. Despite the device iotlb gets
>>>>> re-initialized, the messages are not cleared. Fix that by calling
>>>>> vhost_clear_msg() at the end of vhost_init_device_iotlb().
>>>>>
>>>>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>>>>> ---
>>>>>  drivers/vhost/vhost.c | 1 +
>>>>>  1 file changed, 1 insertion(+)
>>>>>
>>>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>>>>> index 40097826cff0..422a1fdee0ca 100644
>>>>> --- a/drivers/vhost/vhost.c
>>>>> +++ b/drivers/vhost/vhost.c
>>>>> @@ -1751,6 +1751,7 @@ int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled)
>>>>>    }
>>>>>
>>>>>    vhost_iotlb_free(oiotlb);
>>>>> +  vhost_clear_msg(d);
>>>>>
>>>>>    return 0;
>>>>>  }
>>>> Hmm.  Can't messages meanwhile get processes and affect the
>>>> new iotlb?
>>> Isn't the msg processing stopped at the moment this function is called
>>> (VHOST_SET_FEATURES)?
>>>
>>> Thanks
>>>
>>> Eric
>> It's pretty late here I'm not sure.  You tell me what prevents it.
> So the proposed code assumes that Qemu doesn't process device IOTLB
> before VHOST_SET_FEAETURES. Consider there's no reset in the general
> vhost uAPI,  I wonder if it's better to move the clear to device code
> like VHOST_NET_SET_BACKEND. So we can clear it per vq?

OK I will look at this alternative
>
>> BTW vhost_init_device_iotlb gets enabled parameter but ignores
>> it, we really should drop that.
> Yes.
Yes I saw that too. I will send a patch.
>
>> Also, it looks like if features are set with VIRTIO_F_ACCESS_PLATFORM
>> and then cleared, iotlb is not properly cleared - bug?
> Not sure, old IOTLB may still work. But for safety, we need to disable
> device IOTLB in this case.
OK

Thanks

Eric
>
> Thanks
>
>>
>>>>
>>>>> --
>>>>> 2.37.3

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