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

Jason Wang
2022-11-09T03:45:18Z

On Tue, Nov 8, 2022 at 6:17 PM Eric Auger <eric.auger@redhat.com> wrote:
>
> 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@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@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?
> >>> 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?

It depends on where we want to place the vhost_clear_msg(), e.g in
most of the device ioctl, the dev->mutex has been held.

Thanks

>
> 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
>