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Tsirkin" , qemu-level , Markus Armbruster , Stefano Garzarella , Harpreet Singh Anand , Xiao W Wang , Stefan Hajnoczi , Eli Cohen , virtualization@lists.linux-foundation.org, Michael Lilja , Jim Harford , Rob Miller Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Feb 5, 2021 at 4:52 AM Jason Wang wrote: > > > On 2021/2/4 =E4=B8=8B=E5=8D=885:25, Eugenio Perez Martin wrote: > > On Thu, Feb 4, 2021 at 4:14 AM Jason Wang wrote: > >> > >> On 2021/2/2 =E4=B8=8B=E5=8D=886:17, Eugenio Perez Martin wrote: > >>> On Tue, Feb 2, 2021 at 4:31 AM Jason Wang wrote= : > >>>> On 2021/2/1 =E4=B8=8B=E5=8D=884:28, Eugenio Perez Martin wrote: > >>>>> On Mon, Feb 1, 2021 at 7:13 AM Jason Wang wro= te: > >>>>>> On 2021/1/30 =E4=B8=8A=E5=8D=884:54, Eugenio P=C3=A9rez wrote: > >>>>>>> Signed-off-by: Eugenio P=C3=A9rez > >>>>>>> --- > >>>>>>> include/hw/virtio/vhost.h | 1 + > >>>>>>> hw/virtio/vhost.c | 17 +++++++++++++++++ > >>>>>>> 2 files changed, 18 insertions(+) > >>>>>>> > >>>>>>> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.= h > >>>>>>> index 4a8bc75415..fca076e3f0 100644 > >>>>>>> --- a/include/hw/virtio/vhost.h > >>>>>>> +++ b/include/hw/virtio/vhost.h > >>>>>>> @@ -123,6 +123,7 @@ uint64_t vhost_get_features(struct vhost_dev = *hdev, const int *feature_bits, > >>>>>>> void vhost_ack_features(struct vhost_dev *hdev, const int *f= eature_bits, > >>>>>>> uint64_t features); > >>>>>>> bool vhost_has_free_slot(void); > >>>>>>> +struct vhost_dev *vhost_dev_from_virtio(const VirtIODevice *vdev= ); > >>>>>>> > >>>>>>> int vhost_net_set_backend(struct vhost_dev *hdev, > >>>>>>> struct vhost_vring_file *file); > >>>>>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c > >>>>>>> index 28c7d78172..8683d507f5 100644 > >>>>>>> --- a/hw/virtio/vhost.c > >>>>>>> +++ b/hw/virtio/vhost.c > >>>>>>> @@ -61,6 +61,23 @@ bool vhost_has_free_slot(void) > >>>>>>> return slots_limit > used_memslots; > >>>>>>> } > >>>>>>> > >>>>>>> +/* > >>>>>>> + * Get the vhost device associated to a VirtIO device. > >>>>>>> + */ > >>>>>>> +struct vhost_dev *vhost_dev_from_virtio(const VirtIODevice *vdev= ) > >>>>>>> +{ > >>>>>>> + struct vhost_dev *hdev; > >>>>>>> + > >>>>>>> + QLIST_FOREACH(hdev, &vhost_devices, entry) { > >>>>>>> + if (hdev->vdev =3D=3D vdev) { > >>>>>>> + return hdev; > >>>>>>> + } > >>>>>>> + } > >>>>>>> + > >>>>>>> + assert(hdev); > >>>>>>> + return NULL; > >>>>>>> +} > >>>>>> I'm not sure this can work in the case of multiqueue. E.g vhost-ne= t > >>>>>> multiqueue is a N:1 mapping between vhost devics and virtio device= s. > >>>>>> > >>>>>> Thanks > >>>>>> > >>>>> Right. We could add an "vdev vq index" parameter to the function in > >>>>> this case, but I guess the most reliable way to do this is to add a > >>>>> vhost_opaque value to VirtQueue, as Stefan proposed in previous RFC= . > >>>> So the question still, it looks like it's easier to hide the shadow > >>>> virtqueue stuffs at vhost layer instead of expose them to virtio lay= er: > >>>> > >>>> 1) vhost protocol is stable ABI > >>>> 2) no need to deal with virtio stuffs which is more complex than vho= st > >>>> > >>>> Or are there any advantages if we do it at virtio layer? > >>>> > >>> As far as I can tell, we will need the virtio layer the moment we > >>> start copying/translating buffers. > >>> > >>> In this series, the virtio dependency can be reduced if qemu does not > >>> check the used ring _F_NO_NOTIFY flag before writing to irqfd. It > >>> would enable packed queues and IOMMU immediately, and I think the cos= t > >>> should not be so high. In the previous RFC this check was deleted > >>> later anyway, so I think it was a bad idea to include it from the sta= rt. > >> > >> I am not sure I understand here. For vhost, we can still do anything w= e > >> want, e.g accessing guest memory etc. Any blocker that prevent us from > >> copying/translating buffers? (Note that qemu will propagate memory > >> mappings to vhost). > >> > > There is nothing that forbids us to access directly, but if we don't > > reuse the virtio layer functionality we would have to duplicate every > > access function. "Need" was a too strong word maybe :). > > > > In other words: for the shadow vq vring exposed for the device, qemu > > treats it as a driver, and this functionality needs to be added to > > qemu. But for accessing the guest's one do not reuse virtio.c would be > > a bad idea in my opinion. > > > The problem is, virtio.c is not a library and it has a lot of dependency > with other qemu modules basically makes it impossible to be reused at > vhost level. > While virtio.c as a whole has dependencies, I think that the functions needed in the original RFC do not have these dependencies. However I see how to split vring dataplane from virtio device management can benefit. > We can solve this by: > > 1) split the core functions out as a library or > 2) switch to use contrib/lib-vhostuser but needs to decouple UNIX socket > transport > > None of the above looks trivial and they are only device codes. For > shadow virtqueue, we need driver codes as well where no code can be reuse= d. > > As we discussed, we probably need IOVA allocated when forwarding > descriptors between the two virtqueues. So my feeling is we can have our > own codes to start then we can consider whether we can reuse some from > the existing virtio.c or lib-vhostuser. > As I see it, if we develop our own code a lot of it will be copied from current virtio.c, which itself duplicates a lot of contrib/ lib functionality. Maybe it's better to combine your proposals and decouple the vring functions, the vhost transport, and the qemu virtio device management, so other projects can reuse them directly? I still think this can be left for a later series with buffer forwarding on top of this one, do you think they can/should be merged independently? Thanks! > Thanks > > > > > >> Thanks > >> > >> > >>> > >>> > >>> > >>> > >>>> Thanks > >>>> > >>>> > >>>>> I need to take this into account in qmp_x_vhost_enable_shadow_vq to= o. > >>>>> > >>>>>>> + > >>>>>>> static void vhost_dev_sync_region(struct vhost_dev *dev, > >>>>>>> MemoryRegionSection *secti= on, > >>>>>>> uint64_t mfirst, uint64_t = mlast, > > >