[Virtio-fs] (no subject)

Mon Oct 9 10:28:56 UTC 2023

On Sat, Oct 7, 2023 at 4:23 AM Yajun Wu <yajunw at nvidia.com> wrote:
>
>
> On 10/6/2023 6:34 PM, Michael S. Tsirkin wrote:
> > External email: Use caution opening links or attachments
> >
> >
> > On Fri, Oct 06, 2023 at 11:47:55AM +0200, Hanna Czenczek wrote:
> >> On 06.10.23 11:26, Michael S. Tsirkin wrote:
> >>> On Fri, Oct 06, 2023 at 11:15:55AM +0200, Hanna Czenczek wrote:
> >>>> On 06.10.23 10:45, Michael S. Tsirkin wrote:
> >>>>> On Fri, Oct 06, 2023 at 09:48:14AM +0200, Hanna Czenczek wrote:
> >>>>>> On 05.10.23 19:15, Michael S. Tsirkin wrote:
> >>>>>>> On Thu, Oct 05, 2023 at 01:08:52PM -0400, Stefan Hajnoczi wrote:
> >>>>>>>> On Wed, Oct 04, 2023 at 02:58:57PM +0200, Hanna Czenczek wrote:
> >>>>>>>>> There is no clearly defined purpose for the virtio status byte in
> >>>>>>>>> vhost-user: For resetting, we already have RESET_DEVICE; and for virtio
> >>>>>>>>> feature negotiation, we have [GS]ET_FEATURES.  With the REPLY_ACK
> >>>>>>>>> protocol extension, it is possible for SET_FEATURES to return errors
> >>>>>>>>> (SET_PROTOCOL_FEATURES may be called before SET_FEATURES).
> >>>>>>>>>
> >>>>>>>>> As for implementations, SET_STATUS is not widely implemented.  dpdk does
> >>>>>>>>> implement it, but only uses it to signal feature negotiation failure.
> >>>>>>>>> While it does log reset requests (SET_STATUS 0) as such, it effectively
> >>>>>>>>> ignores them, in contrast to RESET_OWNER (which is deprecated, and today
> >>>>>>>>> means the same thing as RESET_DEVICE).
> >>>>>>>>>
> >>>>>>>>> While qemu superficially has support for [GS]ET_STATUS, it does not
> >>>>>>>>> forward the guest-set status byte, but instead just makes it up
> >>>>>>>>> internally, and actually completely ignores what the back-end returns,
> >>>>>>>>> only using it as the template for a subsequent SET_STATUS to add single
> >>>>>>>>> bits to it.  Notably, after setting FEATURES_OK, it never reads it back
> >>>>>>>>> to see whether the flag is still set, which is the only way in which
> >>>>>>>>> dpdk uses the status byte.
> >>>>>>>>>
> >>>>>>>>> As-is, no front-end or back-end can rely on the other side handling this
> >>>>>>>>> field in a useful manner, and it also provides no practical use over
> >>>>>>>>> other mechanisms the vhost-user protocol has, which are more clearly
> >>>>>>>>> defined.  Deprecate it.
> >>>>>>>>>
> >>>>>>>>> Suggested-by: Stefan Hajnoczi <stefanha at redhat.com>
> >>>>>>>>> Signed-off-by: Hanna Czenczek <hreitz at redhat.com>
> >>>>>>>>> ---
> >>>>>>>>>      docs/interop/vhost-user.rst | 28 +++++++++++++++++++++-------
> >>>>>>>>>      1 file changed, 21 insertions(+), 7 deletions(-)
> >>>>>>>> Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
> >>>>>>> SET_STATUS is the only way to signal failure to acknowledge FEATURES_OK.
> >>>>>>> The fact current backends never check errors does not mean they never
> >>>>>>> will. So no, not applying this.
> >>>>>> Can this not be done with REPLY_ACK?  I.e., with the following message
> >>>>>> order:
> >>>>>>
> >>>>>> 1. GET_FEATURES to find out whether VHOST_USER_F_PROTOCOL_FEATURES is
> >>>>>> present
> >>>>>> 2. GET_PROTOCOL_FEATURES to hopefully get VHOST_USER_PROTOCOL_F_REPLY_ACK
> >>>>>> 3. SET_PROTOCOL_FEATURES to set VHOST_USER_PROTOCOL_F_REPLY_ACK
> >>>>>> 4. SET_FEATURES with need_reply
> >>>>>>
> >>>>>> If not, the problem is that qemu has sent SET_STATUS 0 for a while when the
> >>>>>> vCPUs are stopped, which generally seems to request a device reset.  If we
> >>>>>> don’t state at least that SET_STATUS 0 is to be ignored, back-ends that will
> >>>>>> implement SET_STATUS later may break with at least these qemu versions.  But
> >>>>>> documenting that a particular use of the status byte is to be ignored would
> >>>>>> be really strange.
> >>>>>>
> >>>>>> Hanna
> >>>>> Hmm I guess. Though just following virtio spec seems cleaner to me...
> >>>>> vhost-user reconfigures the state fully on start.
> >>>> Not the internal device state, though.  virtiofsd has internal state, and
> >>>> other devices like vhost-gpu back-ends would probably, too.
> >>>>
> >>>> Stefan has recently sent a series
> >>>> (https://lists.nongnu.org/archive/html/qemu-devel/2023-10/msg00709.html) to
> >>>> put the reset (RESET_DEVICE) into virtio_reset() (when we really need a
> >>>> reset).
> >>>>
> >>>> I really don’t like our current approach with the status byte. Following the
> >>>> virtio specification to me would mean that the guest directly controls this
> >>>> byte, which it does not.  qemu makes up values as it deems appropriate, and
> >>>> this includes sending a SET_STATUS 0 when the guest is just paused, i.e.
> >>>> when the guest really doesn’t want a device reset.
> >>>>
> >>>> That means that qemu does not treat this as a virtio device field (because
> >>>> that would mean exposing it to the guest driver), but instead treats it as
> >>>> part of the vhost(-user) protocol.  It doesn’t feel right to me that we use
> >>>> a virtio-defined feature for communication on the vhost level, i.e. between
> >>>> front-end and back-end, and not between guest driver and device.  I think
> >>>> all vhost-level protocol features should be fully defined in the vhost-user
> >>>> specification, which REPLY_ACK is.
> >>> Hmm that makes sense. Maybe we should have done what stefan's patch
> >>> is doing.
> >>>
> >>> Do look at the original commit that introduced it to understand why
> >>> it was added.
> >> I don’t understand why this was added to the stop/cont code, though.  If it
> >> is time consuming to make these changes, why are they done every time the VM
> >> is paused
> >> and resumed?  It makes sense that this would be done for the initial
> >> configuration (where a reset also wouldn’t hurt), but here it seems wrong.
> >>
> >> (To be clear, a reset in the stop/cont code is wrong, because it breaks
> >> stateful devices.)
> >>
> >> Also, note the newer commits 6f8be29ec17 and c3716f260bf.  The reset as
> >> originally introduced was wrong even for non-stateful devices, because it
> >> occurred before we fetched the state (vring indices) so we could restore it
> >> later.  I don’t know how 923b8921d21 was tested, but if the back-end used
> >> for testing implemented SET_STATUS 0 as a reset, it could not have survived
> >> either migration or a stop/cont in general, because the vring indices would
> >> have been reset to 0.
> >>
> >> What I’m saying is, 923b8921d21 introduced SET_STATUS calls that broke all
> >> devices that would implement them as per virtio spec, and even today it’s
> >> broken for stateful devices.  The mentioned performance issue is likely
> >> real, but we can’t address it by making up SET_STATUS calls that are wrong.
> >>
> >> I concede that I didn’t think about DRIVER_OK.  Personally, I would do all
> >> final configuration that would happen upon a DRIVER_OK once the first vring
> >> is started (i.e. receives a kick).  That has the added benefit of being
> >> asynchronous because it doesn’t block any vhost-user messages (which are
> >> synchronous, and thus block downtime).
> >>
> >> Hanna
> >
> > For better or worse kick is per ring. It's out of spec to start rings
> > that were not kicked but I guess you could do configuration ...
> > Seems somewhat asymmetrical though.
> >
> > Let's wait until next week, hopefully Yajun Wu will answer.
> The main motivation of adding VHOST_USER_SET_STATUS is to let backend
> DPDK know
> when DRIVER_OK bit is valid. It's an indication of all VQ configuration
> has sent,
> otherwise DPDK has to rely on first queue pair is ready, then
> receiving/applying
> VQ configuration one by one.
>
> During live migration, configuring VQ one by one is very time consuming.
> For VIRTIO
> net vDPA, HW needs to know how many VQs are enabled to set
> RSS(Receive-Side Scaling).
>
> If you don’t want SET_STATUS message, backend can remove protocol
> feature bit
> VHOST_USER_PROTOCOL_F_STATUS.
> DPDK is ignoring SET_STATUS 0, but using GET_VRING_BASE to do device
> close/reset.

This is incorrect, resetting the device on GET_VRING_BASE breaks
the stop/cont. Since you don't want to reset the VQs on stop/cont.

>
> I'm not involved in discussion about adding SET_STATUS in Vhost
> protocol. This feature
> is essential for vDPA(same as vhost-vdpa implements VHOST_VDPA_SET_STATUS).
>
> Thanks,
> Yajun
> >
> >>>> Now, we could hand full control of the status byte to the guest, and that
> >>>> would make me content.  But I feel like that doesn’t really work, because
> >>>> qemu needs to intercept the status byte anyway (it needs to know when there
> >>>> is a reset, probably wants to know when the device is configured, etc.), so
> >>>> I don’t think having the status byte in vhost-user really gains us much when
> >>>> qemu could translate status byte changes to/from other vhost-user commands.
> >>>>
> >>>> Hanna
> >>> well it intercepts it but I think it could pass it on unchanged.
> >>>
> >>>
> >>>>> I guess symmetry was the
> >>>>> point. So I don't see why SET_STATUS 0 has to be ignored.
> >>>>>
> >>>>>
> >>>>> SET_STATUS was introduced by:
> >>>>>
> >>>>> commit 923b8921d210763359e96246a58658ac0db6c645
> >>>>> Author: Yajun Wu <yajunw at nvidia.com>
> >>>>> Date:   Mon Oct 17 14:44:52 2022 +0800
> >>>>>
> >>>>>        vhost-user: Support vhost_dev_start
> >>>>>
> >>>>> CC the author.
> >>>>>
>
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