Message-ID: <CACGkMEtZ_oEydqsvJh0-eaDh4q_KHq9fLPg1uy-W1m7K_g+1-w@mail.gmail.com>
Date:   Wed, 18 Oct 2023 15:00:44 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Si-Wei Liu <si-wei.liu@...cle.com>
Cc:     Eugenio Perez Martin <eperezma@...hat.com>, mst@...hat.com,
        xuanzhuo@...ux.alibaba.com, dtatulea@...dia.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] vhost-vdpa: reset vendor specific mapping to initial
 state in .release

On Wed, Oct 18, 2023 at 1:27 PM Jason Wang <jasowang@...hat.com> wrote:
>
> On Wed, Oct 18, 2023 at 12:36 PM Si-Wei Liu <si-wei.liu@...cle.com> wrote:
> >
> >
> >
> > On 10/16/2023 7:35 PM, Jason Wang wrote:
> > > On Tue, Oct 17, 2023 at 4:30 AM Si-Wei Liu <si-wei.liu@...cle.com> wrote:
> > >>
> > >>
> > >> On 10/16/2023 4:28 AM, Eugenio Perez Martin wrote:
> > >>> On Mon, Oct 16, 2023 at 8:33 AM Jason Wang <jasowang@...hat.com> wrote:
> > >>>> On Fri, Oct 13, 2023 at 3:36 PM Si-Wei Liu <si-wei.liu@...cle.com> wrote:
> > >>>>>
> > >>>>> On 10/12/2023 8:01 PM, Jason Wang wrote:
> > >>>>>> On Tue, Oct 10, 2023 at 5:05 PM Si-Wei Liu <si-wei.liu@...cle.com> wrote:
> > >>>>>>> Devices with on-chip IOMMU or vendor specific IOTLB implementation
> > >>>>>>> may need to restore iotlb mapping to the initial or default state
> > >>>>>>> using the .reset_map op, as it's desirable for some parent devices
> > >>>>>>> to solely manipulate mappings by its own, independent of virtio device
> > >>>>>>> state. For instance, device reset does not cause mapping go away on
> > >>>>>>> such IOTLB model in need of persistent mapping. Before vhost-vdpa
> > >>>>>>> is going away, give them a chance to reset iotlb back to the initial
> > >>>>>>> state in vhost_vdpa_cleanup().
> > >>>>>>>
> > >>>>>>> Signed-off-by: Si-Wei Liu <si-wei.liu@...cle.com>
> > >>>>>>> ---
> > >>>>>>>     drivers/vhost/vdpa.c | 16 ++++++++++++++++
> > >>>>>>>     1 file changed, 16 insertions(+)
> > >>>>>>>
> > >>>>>>> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> > >>>>>>> index 851535f..a3f8160 100644
> > >>>>>>> --- a/drivers/vhost/vdpa.c
> > >>>>>>> +++ b/drivers/vhost/vdpa.c
> > >>>>>>> @@ -131,6 +131,15 @@ static struct vhost_vdpa_as *vhost_vdpa_find_alloc_as(struct vhost_vdpa *v,
> > >>>>>>>            return vhost_vdpa_alloc_as(v, asid);
> > >>>>>>>     }
> > >>>>>>>
> > >>>>>>> +static void vhost_vdpa_reset_map(struct vhost_vdpa *v, u32 asid)
> > >>>>>>> +{
> > >>>>>>> +       struct vdpa_device *vdpa = v->vdpa;
> > >>>>>>> +       const struct vdpa_config_ops *ops = vdpa->config;
> > >>>>>>> +
> > >>>>>>> +       if (ops->reset_map)
> > >>>>>>> +               ops->reset_map(vdpa, asid);
> > >>>>>>> +}
> > >>>>>>> +
> > >>>>>>>     static int vhost_vdpa_remove_as(struct vhost_vdpa *v, u32 asid)
> > >>>>>>>     {
> > >>>>>>>            struct vhost_vdpa_as *as = asid_to_as(v, asid);
> > >>>>>>> @@ -140,6 +149,13 @@ static int vhost_vdpa_remove_as(struct vhost_vdpa *v, u32 asid)
> > >>>>>>>
> > >>>>>>>            hlist_del(&as->hash_link);
> > >>>>>>>            vhost_vdpa_iotlb_unmap(v, &as->iotlb, 0ULL, 0ULL - 1, asid);
> > >>>>>>> +       /*
> > >>>>>>> +        * Devices with vendor specific IOMMU may need to restore
> > >>>>>>> +        * iotlb to the initial or default state which is not done
> > >>>>>>> +        * through device reset, as the IOTLB mapping manipulation
> > >>>>>>> +        * could be decoupled from the virtio device life cycle.
> > >>>>>>> +        */
> > >>>>>> Should we do this according to whether IOTLB_PRESIST is set?
> > >>>>> Well, in theory this seems like so but it's unnecessary code change
> > >>>>> actually, as that is the way how vDPA parent behind platform IOMMU works
> > >>>>> today, and userspace doesn't break as of today. :)
> > >>>> Well, this is one question I've ever asked before. You have explained
> > >>>> that one of the reason that we don't break userspace is that they may
> > >>>> couple IOTLB reset with vDPA reset as well. One example is the Qemu.
> > >>>>
> > >>>>> As explained in previous threads [1][2], when IOTLB_PERSIST is not set
> > >>>>> it doesn't necessarily mean the iotlb will definitely be destroyed
> > >>>>> across reset (think about the platform IOMMU case), so userspace today
> > >>>>> is already tolerating enough with either good or bad IOMMU.
> > > I'm confused, how to define tolerating here?
> >
> > Tolerating defined as QEMU has to proactively unmap before reset just to
> > workaround the driver bug (on-chip maps out of sync), unconditionally
> > for platform or on-chip. While we all know it doesn't have to do so for
> > platform IOMMU, though userspace has no means to distinguish. That said,
> > userspace is sacrificing reset time performance on platform IOMMU setup
> > just for working around buggy implementation in the other setup.
>
> Ok, so what you actually mean is that userspace can tolerate the "bug"
> with the performance penalty.
>
>
> >
> > > For example, if it has tolerance, why bother?
> > I'm not sure I get the question. But I think userspace is compromising
> > because of buggy implementation in a few drivers doesn't mean we should
> > uniformly enforce such behavior for all set_map/dma_map implementations.
>
> This is not my point. I meant, we can fix we need a negotiation in
> order to let some "buggy" old user space to survive from the changes.
>
> >
> > >
> > >>>> This code of
> > >>>>> not checking IOTLB_PERSIST being set is intentional, there's no point to
> > >>>>> emulate bad IOMMU behavior even for older userspace (with improper
> > >>>>> emulation to be done it would result in even worse performance).
> > > I can easily imagine a case:
> > >
> > > The old Qemu that works only with a setup like mlx5_vdpa.
> > Noted, seems to me there's no such case of a userspace implementation
> > that only works with mlx5_vdpa or its friends, but doesn't work with the
> > others e.g. platform IOMMU, or well behaving on-chip IOMMU
> > implementations.
>
> It's not hard to think of a case where:
>
> 1) the environment has mlx5_vdpa only
> 2) kernel doc can't have endless details, so when developing
> application, the author notice IOTLB is cleared during reset
>
> > The Unmap+remap trick around vdpa reset works totally
> > fine for platform IOMMU, except with sub-optimal performance. Other than
> > this trick, I cannot easily think of other means or iotlb message
> > sequence for userspace to recover the bogus state and make iotlb back to
> > work again after reset.
>
> Yes for sure, but we can't audit every user space, no?
>
> > Are we talking about hypnosis that has no real
> > basis to exist in the real world?
>
> Instead of trying to answer these hard questions, I would go another
> way. That is, stick to the old behaviour when IOTLB_PRESISIT is not
> set by the backend. This is much easier.
>
> >
> > >   If we do
> > > this without a negotiation, IOTLB will not be clear but the Qemu will
> > > try to re-program the IOTLB after reset. Which will break?
> > >
> > > 1) stick the exact old behaviour with just one line of check
> > It's not just one line of check here, the old behavior emulation has to
> > be done as Eugenio illustrated in the other email.
>
> For vhost-vDPA it's just
>
> if (IOTLB_PERSIST is acked by userspace)
>     reset_map()
>
> For parent, it's somehow similar:
>
> during .reset()
>
> if (IOTLB_PERSIST is not acked by userspace)
>         reset_vendor_mappings()
>
> Anything I missed here?
>
> > In addition, the
> > emulation has to limit to those buggy drivers as I don't feel this
> > emulation should apply uniformly to all future set_map/dma_map
> > implementations.
>
> Unfortunately, it's a must to stick to ABI. I agree it's a mess but we
> don't have a better choice. Or we can fail the probe if userspace
> doesn't ack this feature.

Antoher idea we can just do the following in vhost_vdpa reset?

config->reset()
if (IOTLB_PERSIST is not set) {
    config->reset_map()
}

Then we don't have the burden to maintain them in the parent?

Thanks

>
> > > 2) audit all the possible cases to avoid a one line of code
> > >
> > > 1) seems much easier than 2)
> > You see it's more than just one line of code, and I'm uncertain if the
> > additional complexity is warranted or necessary, particularly if added
> > this piece of compatibility code will linger for quite a long time.
>
> This is a must as long as it can be noticed by userspace. Doing
> something conservative makes more sense to me.
>
> > Instead of adding hypothetical code change for no specific good reason
> > and no real use case,
>
> It's not adding something new or new behaviours, it's just making the
> IOTLB reset conditional based on vDPA reset.
>
> > I'd like to add the code when we find out a
> > specific use case that may get impacted or already being affected,
>
> It doesn't conflict with what you proposed here. Old behaviours have
> their users, no?
>
> > then
> > we will have good understanding how to code up the fix and emulate
> > properly for compatibility, while not affecting other good implementations.
>
> The issue is, even if we can't find a userspace now. It doesn't mean
> we can't have one in the future. Then it might be too late or too
> tricky to fix them. We had a lot of lessons in the past.
>
> Thanks
>
> >
> > Thanks,
> > -Siwe/i/
> >
> > >
> > >>>> For two reasons:
> > >>>>
> > >>>> 1) backend features need acked by userspace this is by design
> > >>>> 2) keep the odd behaviour seems to be more safe as we can't audit
> > >>>> every userspace program
> > >>>>
> > >>> The old behavior (without flag ack) cannot be trusted already, as:
> > > Possibly but the point is to unbreak userspace no matter how weird the
> > > behaviour we've ever had.
> > >
> > >>> * Devices using platform IOMMU (in other words, not implementing
> > >>> neither .set_map nor .dma_map) does not unmap memory at virtio reset.
> > >>> * Devices that implement .set_map or .dma_map (vdpa_sim, mlx5) do
> > >>> reset IOTLB, but in their parent ops (vdpasim_do_reset, prune_iotlb
> > >>> called from mlx5_vdpa_reset). With vdpa_sim patch removing the reset,
> > >>> now all backends work the same as far as I know., which was (and is)
> > >>> the way devices using the platform IOMMU works.
> > >>>
> > >>> The difference in behavior did not matter as QEMU unmaps all the
> > >>> memory unregistering the memory listener at vhost_vdpa_dev_start(...,
> > >>> started = false),
> > >> Exactly. It's not just QEMU, but any (older) userspace manipulates
> > >> mappings through the vhost-vdpa iotlb interface has to unmap all
> > >> mappings to workaround the vdpa parent driver bug.
> > > Just to clarify, from userspace, it's the (odd) behaviour of the current uAPI.
> > >
> > >> If they don't do
> > >> explicit unmap, it would cause state inconsistency between vhost-vdpa
> > >> and parent driver, then old mappings can't be restored, and new mapping
> > >> can be added to iotlb after vDPA reset. There's no point to preserve
> > >> this broken and inconsistent behavior between vhost-vdpa and parent
> > >> driver, as userspace doesn't care at all!
> > > It's a userspace notice change so we can't fix it silently:
> > >
> > > https://lkml.org/lkml/2012/12/23/75
> > >
> > > Another example which is related to vhost-vDPA:
> > >
> > > https://lore.kernel.org/netdev/20230927140544.205088-1-eric.auger@redhat.com/T/
> > >
> > > Thanks
> > >
> > >>> but the backend acknowledging this feature flag
> > >>> allows QEMU to make sure it is safe to skip this unmap & map in the
> > >>> case of vhost stop & start cycle.
> > >>>
> > >>> In that sense, this feature flag is actually a signal for userspace to
> > >>> know that the bug has been solved.
> > >> Right, I couldn't say it better than you do, thanks! The feature flag is
> > >> more of an unusual means to indicating kernel bug having been fixed,
> > >> rather than introduce a new feature or new kernel behavior ending up in
> > >> change of userspace's expectation.
> > >>
> > >>> Not offering it indicates that
> > >>> userspace cannot trust the kernel will retain the maps.
> > >>>
> > >>> Si-Wei or Dragos, please correct me if I've missed something. Feel
> > >>> free to use the text in case you find more clear in doc or patch log.
> > >> Sure, will do, thank you! Will post v2 adding these to the log.
> > >>
> > >> Thanks,
> > >> -Siwei
> > >>
> > >>
> > >>
> > >>> Thanks!
> > >>>
> > >>>> Thanks
> > >>>>
> > >>>>> I think
> > >>>>> the purpose of the IOTLB_PERSIST flag is just to give userspace 100%
> > >>>>> certainty of persistent iotlb mapping not getting lost across vdpa reset.
> > >>>>>
> > >>>>> Thanks,
> > >>>>> -Siwei
> > >>>>>
> > >>>>> [1]
> > >>>>> https://lore.kernel.org/virtualization/9f118fc9-4f6f-dd67-a291-be78152e47fd@oracle.com/
> > >>>>> [2]
> > >>>>> https://lore.kernel.org/virtualization/3364adfd-1eb7-8bce-41f9-bfe5473f1f2e@oracle.com/
> > >>>>>>     Otherwise
> > >>>>>> we may break old userspace.
> > >>>>>>
> > >>>>>> Thanks
> > >>>>>>
> > >>>>>>> +       vhost_vdpa_reset_map(v, asid);
> > >>>>>>>            kfree(as);
> > >>>>>>>
> > >>>>>>>            return 0;
> > >>>>>>> --
> > >>>>>>> 1.8.3.1
> > >>>>>>>
> >

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