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 <stefa...@redhat.com> > > > > > > > > Signed-off-by: Hanna Czenczek <hre...@redhat.com> > > > > > > > > --- > > > > > > > > docs/interop/vhost-user.rst | 28 > > > > > > > > +++++++++++++++++++++------- > > > > > > > > 1 file changed, 21 insertions(+), 7 deletions(-) > > > > > > > Reviewed-by: Stefan Hajnoczi <stefa...@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. > > > 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 <yaj...@nvidia.com> > > > > Date: Mon Oct 17 14:44:52 2022 +0800 > > > > > > > > vhost-user: Support vhost_dev_start > > > > > > > > CC the author. > > > >
