Re: [dpdk-dev] [PATCH] vhost: Clean iotlb cache on vring stop

[Eugenio Perez Martin <eperezma@redhat.com>]

On Mon, Aug 30, 2021 at 1:58 PM Xia, Chenbo <chenbo.xia@intel.com> wrote:
>
> Hi Eugenio,
>

Hi Chenbo,

> > -----Original Message-----
> > From: Eugenio Pérez <eperezma@redhat.com>
> > Sent: Saturday, August 28, 2021 12:13 AM
> > To: Maxime Coquelin <maxime.coquelin@redhat.com>; Xia, Chenbo
> > <chenbo.xia@intel.com>
> > Cc: dev@dpdk.org; Pei Zhang <pezhang@redhat.com>; Jason Wang
> > <jasowang@redhat.com>
> > Subject: [PATCH] vhost: Clean iotlb cache on vring stop
>
> Clean -> clean
>

Is that something I need to send a new revision for, or is it ok to
apply on the maintainer side?

> >
> > Old IOVA cache entries are left when there is a change on virtio driver
> > in VM. In case that all these old entries have iova addresses lesser
> > than new iova entries, vhost code will need to iterate all the cache to
> > find the new ones. In case of just a new iova entry needed for the new
> > translations, this condition will last forever.
> >
> > This has been observed in virtio-net to testpmd's vfio-pci driver
> > transition, reducing the performance from more than 10Mpps to less than
> > 0.07Mpps if the hugepage address was higher than the networking
> > buffers. Since all new buffers are contained in this new gigantic page,
> > vhost needs to scan IOTLB_CACHE_SIZE - 1 for each translation at worst.
>
> I'm curious why QEMU will not invalidate iotlb when virtio-net driver is removed
> (dma region should be unmapped).
>

I'm going to investigate this more, but qemu iommu notifier callback
(vhost_iommu_unmap_notify) is never called through all the test. Also,
guest kernel code calls dma_unmap_page for each buffer and vqs, but it
never generates an iotlb flush.

Or do you mean that qemu should also flush all iotlb entries on vhost
device stop?

> And since the perf drop is huge, why not cc to stable and add fix tag?
>

I was not sure if it was worth it to backport, but I would say that
the issue can be reproduced with enough bad luck. Since translations
have always been saved in a linked list:

Fixes: d012d1f293f4 ("vhost: add IOTLB helper functions")

Same question as before, if no changes to the code are needed for the
patch, do I need to send a second revision?

Thanks!


> Thanks,
> Chenbo
>
> >
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > Reported-by: Pei Zhang <pezhang@redhat.com>
> > ---
> >  lib/vhost/vhost_user.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/lib/vhost/vhost_user.c b/lib/vhost/vhost_user.c
> > index 29a4c9af60..7de48f5333 100644
> > --- a/lib/vhost/vhost_user.c
> > +++ b/lib/vhost/vhost_user.c
> > @@ -2113,6 +2113,8 @@ vhost_user_get_vring_base(struct virtio_net **pdev,
> >       msg->size = sizeof(msg->payload.state);
> >       msg->fd_num = 0;
> >
> > +     vhost_user_iotlb_flush_all(vq);
> > +
> >       vring_invalidate(dev, vq);
> >
> >       return RTE_VHOST_MSG_RESULT_REPLY;
> > --
> > 2.27.0
>