On Thu, Jun 8, 2023 at 5:21 PM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote: > > On Thu, Jun 08, 2023 at 05:00:00PM +0800, Jason Wang wrote: > >On Thu, Jun 8, 2023 at 4:00 PM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote: > >> > >> On Thu, Jun 08, 2023 at 03:46:00PM +0800, Jason Wang wrote: > >> > >> [...] > >> > >> >> > > > > I have a question though, what if down the road there > >> >> > > > > is a new feature that needs more changes? It will be > >> >> > > > > broken too just like PACKED no? > >> >> > > > > Shouldn't vdpa have an allowlist of features it knows how > >> >> > > > > to support? > >> >> > > > > >> >> > > > It looks like we had it, but we took it out (by the way, we were > >> >> > > > enabling packed even though we didn't support it): > >> >> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6234f80574d7569444d8718355fa2838e92b158b > >> >> > > > > >> >> > > > The only problem I see is that for each new feature we have to modify > >> >> > > > the kernel. > >> >> > > > Could we have new features that don't require handling by vhost-vdpa? > >> >> > > > > >> >> > > > Thanks, > >> >> > > > Stefano > >> >> > > > >> >> > > Jason what do you say to reverting this? > >> >> > > >> >> > I may miss something but I don't see any problem with vDPA core. > >> >> > > >> >> > It's the duty of the parents to advertise the features it has. For example, > >> >> > > >> >> > 1) If some kernel version that is packed is not supported via > >> >> > set_vq_state, parents should not advertise PACKED features in this > >> >> > case. > >> >> > 2) If the kernel has support packed set_vq_state(), but it's emulated > >> >> > cvq doesn't support, parents should not advertise PACKED as well > >> >> > > >> >> > If a parent violates the above 2, it looks like a bug of the parents. > >> >> > > >> >> > Thanks > >> >> > >> >> Yes but what about vhost_vdpa? Talking about that not the core. > >> > > >> >Not sure it's a good idea to workaround parent bugs via vhost-vDPA. > >> > >> Sorry, I'm getting lost... > >> We were talking about the fact that vhost-vdpa doesn't handle > >> SET_VRING_BASE/GET_VRING_BASE ioctls well for packed virtqueue before > >> that series [1], no? > >> > >> The parents seem okay, but maybe I missed a few things. > >> > >> [1] https://lore.kernel.org/virtualization/20230424225031.18947-1-shannon.nelson@xxxxxxx/ > > > >Yes, more below. > > > >> > >> > > >> >> Should that not have a whitelist of features > >> >> since it interprets ioctls differently depending on this? > >> > > >> >If there's a bug, it might only matter the following setup: > >> > > >> >SET_VRING_BASE/GET_VRING_BASE + VDUSE. > >> > > >> >This seems to be broken since VDUSE was introduced. If we really want > >> >to backport something, it could be a fix to filter out PACKED in > >> >VDUSE? > >> > >> mmm it doesn't seem to be a problem in VDUSE, but in vhost-vdpa. > >> I think VDUSE works fine with packed virtqueue using virtio-vdpa > >> (I haven't tried), so why should we filter PACKED in VDUSE? > > > >I don't think we need any filtering since: > > > >PACKED features has been advertised to userspace via uAPI since > >6234f80574d7569444d8718355fa2838e92b158b. Once we relax in uAPI, it > >would be very hard to restrict it again. For the userspace that tries > >to negotiate PACKED: > > > >1) if it doesn't use SET_VRING_BASE/GET_VRING_BASE, everything works well > >2) if it uses SET_VRING_BASE/GET_VRING_BASE. it might fail or break silently > > > >If we backport the fixes to -stable, we may break the application at > >least in the case 1). > > Okay, I see now, thanks for the details! > > Maybe instead of "break silently", we can return an explicit error for > SET_VRING_BASE/GET_VRING_BASE in stable branches. > But if there are not many cases, we can leave it like that. A second thought, if we need to do something for stable. is it better if we just backport Shannon's series to stable? > > I was just concerned about how does the user space understand that it > can use SET_VRING_BASE/GET_VRING_BASE for PACKED virtqueues in a given > kernel or not. My understanding is that if packed is advertised, the application should assume SET/GET_VRING_BASE work. Thanks > > Thanks, > Stefano >
