Virtualization - Nov 2022 - [RFC] vhost: Clear the pending messages on vhost_init_device_iotlb()

On Tue, Nov 08, 2022 at 05:13:50PM +0800, Jason Wang
wrote:
> On Tue, Nov 8, 2022 at 4:56 PM Michael S. Tsirkin <mst at redhat.com>
wrote:
> >
> > On Tue, Nov 08, 2022 at 11:09:36AM +0800, Jason Wang wrote:
> > > On Tue, Nov 8, 2022 at 7:06 AM Michael S. Tsirkin <mst at
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 at
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?
> >
> > Hmm this makes no sense to me. iommu sits between backend
> > and frontend. Tying one to another is going to backfire.
> 
> I think we need to emulate what real devices are doing. Device should
> clear the page fault message during reset, so the driver won't read
> anything after reset. But we don't have a per device stop or reset
> message for vhost-net. That's why the VHOST_NET_SET_BACKEND came into
> my mind.
That's not a reset message. Userspace can switch backends at will.
I guess we could check when backend is set to -1.
It's a hack but might work.
> >
> > I'm thinking more along the lines of doing everything
> > under iotlb_lock.
> 
> I think the problem is we need to find a proper place to clear the
> message. So I don't get how iotlb_lock can help: the message could be
> still read from user space after the backend is set to NULL.
> 
> Thanks
Well I think the real problem is this.

vhost_net_set_features does:

        if ((features & (1ULL << VIRTIO_F_ACCESS_PLATFORM))) {
                if (vhost_init_device_iotlb(&n->dev, true))
                        goto out_unlock;
        }


so we get a new iotlb each time features are set.

But features can be changes while device is running.
E.g.
	VHOST_F_LOG_ALL


Let's just say this hack of reusing feature bits for backend
was not my brightest idea :(




> >
> >
> >
> > > >
> > > > BTW vhost_init_device_iotlb gets enabled parameter but
ignores
> > > > it, we really should drop that.
> > >
> > > Yes.
> > >
> > > >
> > > > 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.
> > >
> > > Thanks
> > >
> > > >
> > > >
> > > > > >
> > > > > >
> > > > > >> --
> > > > > >> 2.37.3
> > > >
> >

Hi Michael, Jason,

On 11/8/22 10:31, Michael S. Tsirkin wrote:
> On Tue, Nov 08, 2022 at 05:13:50PM +0800, Jason Wang wrote:
>> On Tue, Nov 8, 2022 at 4:56 PM Michael S. Tsirkin <mst at
redhat.com> wrote:
>>> On Tue, Nov 08, 2022 at 11:09:36AM +0800, Jason Wang wrote:
>>>> On Tue, Nov 8, 2022 at 7:06 AM Michael S. Tsirkin <mst at
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 at
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?
>>> Hmm this makes no sense to me. iommu sits between backend
>>> and frontend. Tying one to another is going to backfire.
>> I think we need to emulate what real devices are doing. Device should
>> clear the page fault message during reset, so the driver won't read
>> anything after reset. But we don't have a per device stop or reset
>> message for vhost-net. That's why the VHOST_NET_SET_BACKEND came
into
>> my mind.
> That's not a reset message. Userspace can switch backends at will.
> I guess we could check when backend is set to -1.
> It's a hack but might work.
>
>>> I'm thinking more along the lines of doing everything
>>> under iotlb_lock.
>> I think the problem is we need to find a proper place to clear the
>> message. So I don't get how iotlb_lock can help: the message could
be
>> still read from user space after the backend is set to NULL.
>>
>> Thanks
> Well I think the real problem is this.
>
> vhost_net_set_features does:
>
>         if ((features & (1ULL << VIRTIO_F_ACCESS_PLATFORM))) {
>                 if (vhost_init_device_iotlb(&n->dev, true))
>                         goto out_unlock;
>         }
>
>
> so we get a new iotlb each time features are set.
>
> But features can be changes while device is running.
> E.g.
> 	VHOST_F_LOG_ALL
>
>
> Let's just say this hack of reusing feature bits for backend
> was not my brightest idea :(
>
Isn't vhost_init_device_iotlb() racy then, as d->iotlb is first updated
with niotlb and later d->vqs[i]->iotlb is updated with niotlb. What does
garantee this is done atomically?

Shouldn't we hold the dev->mutex to make all the sequence atomic and
include vhost_clear_msg()?? Can't the vhost_clear_msg() take the dev lock?

Thanks

Eric
>
>
>
>>>
>>>
>>>>> BTW vhost_init_device_iotlb gets enabled parameter but
ignores
>>>>> it, we really should drop that.
>>>> Yes.
>>>>
>>>>> 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.
>>>>
>>>> Thanks
>>>>
>>>>>
>>>>>>>
>>>>>>>> --
>>>>>>>> 2.37.3

On Tue, Nov 8, 2022 at 5:31 PM Michael S. Tsirkin <mst at redhat.com>
wrote:
>
> On Tue, Nov 08, 2022 at 05:13:50PM +0800, Jason Wang wrote:
> > On Tue, Nov 8, 2022 at 4:56 PM Michael S. Tsirkin <mst at
redhat.com> wrote:
> > >
> > > On Tue, Nov 08, 2022 at 11:09:36AM +0800, Jason Wang wrote:
> > > > On Tue, Nov 8, 2022 at 7:06 AM Michael S. Tsirkin <mst at
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
at 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?
> > >
> > > Hmm this makes no sense to me. iommu sits between backend
> > > and frontend. Tying one to another is going to backfire.
> >
> > I think we need to emulate what real devices are doing. Device should
> > clear the page fault message during reset, so the driver won't
read
> > anything after reset. But we don't have a per device stop or reset
> > message for vhost-net. That's why the VHOST_NET_SET_BACKEND came
into
> > my mind.
>
> That's not a reset message. Userspace can switch backends at will.
> I guess we could check when backend is set to -1.
> It's a hack but might work.
Yes, that's what I meant actually.
>
> > >
> > > I'm thinking more along the lines of doing everything
> > > under iotlb_lock.
> >
> > I think the problem is we need to find a proper place to clear the
> > message. So I don't get how iotlb_lock can help: the message could
be
> > still read from user space after the backend is set to NULL.
> >
> > Thanks
>
> Well I think the real problem is this.
>
> vhost_net_set_features does:
>
>         if ((features & (1ULL << VIRTIO_F_ACCESS_PLATFORM))) {
>                 if (vhost_init_device_iotlb(&n->dev, true))
>                         goto out_unlock;
>         }
>
>
> so we get a new iotlb each time features are set.
Right, but this looks like another independent issue that needs to be fixed.
>
> But features can be changes while device is running.
> E.g.
>         VHOST_F_LOG_ALL
>
>
> Let's just say this hack of reusing feature bits for backend
> was not my brightest idea :(
>
Probably :)

Thanks
>
>
>
>
> > >
> > >
> > >
> > > > >
> > > > > BTW vhost_init_device_iotlb gets enabled parameter but
ignores
> > > > > it, we really should drop that.
> > > >
> > > > Yes.
> > > >
> > > > >
> > > > > 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.
> > > >
> > > > Thanks
> > > >
> > > > >
> > > > >
> > > > > > >
> > > > > > >
> > > > > > >> --
> > > > > > >> 2.37.3
> > > > >
> > >
>