Messages in this thread | | | Date | Thu, 8 Jun 2023 11:47:56 +0200 | From | Stefano Garzarella <> | Subject | Re: [PATCH] vhost-vdpa: filter VIRTIO_F_RING_PACKED feature |
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On Thu, Jun 08, 2023 at 05:29:58PM +0800, Jason Wang wrote: >On Thu, Jun 8, 2023 at 5:21 PM Stefano Garzarella <sgarzare@redhat.com> 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@redhat.com> 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@amd.com/ >> > >> >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 tried to look at it, but it looks like we have to backport quite a few patches, I wrote a few things here:
https://lore.kernel.org/virtualization/32ejjuvhvcicv7wjuetkv34qtlpa657n4zlow4eq3fsi2twozk@iqnd2t5tw2an/
But if you think it's the best way, though, we can take a better look at how many patches are to backport and whether it's risky or not.
> >> >> 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. >
Same here. So as an alternative to backporting a large set of patches, I proposed to completely disable packed for stable branches where vhost-vdpa IOCTLs doesn't support them very well.
Thanks, Stefano
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