Re: [PATCH 1/2] vdpa_sim: not reset state in vdpasim_queue_ready

Re: [PATCH 1/2] vdpa_sim: not reset state in vdpasim_queue_ready

[Jason Wang]

On Thu, Jan 19, 2023 at 12:44 AM Eugenio Pérez <eperezma@redhat.com> wrote:
>
> vdpasim_queue_ready calls vringh_init_iotlb, which resets split indexes.
> But it can be called after setting a ring base with
> vdpasim_set_vq_state.
>
> Fix it by stashing them. They're still resetted in vdpasim_vq_reset.
>
> This was discovered and tested live migrating the vdpa_sim_net device.
>
> Fixes: 2c53d0f64c06 ("vdpasim: vDPA device simulator")
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>  drivers/vdpa/vdpa_sim/vdpa_sim.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.c b/drivers/vdpa/vdpa_sim/vdpa_sim.c
> index cb88891b44a8..8839232a3fcb 100644
> --- a/drivers/vdpa/vdpa_sim/vdpa_sim.c
> +++ b/drivers/vdpa/vdpa_sim/vdpa_sim.c
> @@ -66,6 +66,7 @@ static void vdpasim_vq_notify(struct vringh *vring)
>  static void vdpasim_queue_ready(struct vdpasim *vdpasim, unsigned int idx)
>  {
>         struct vdpasim_virtqueue *vq = &vdpasim->vqs[idx];
> +       uint16_t last_avail_idx = vq->vring.last_avail_idx;
>
>         vringh_init_iotlb(&vq->vring, vdpasim->features, vq->num, false,
>                           (struct vring_desc *)(uintptr_t)vq->desc_addr,
> @@ -74,6 +75,7 @@ static void vdpasim_queue_ready(struct vdpasim *vdpasim, unsigned int idx)
>                           (struct vring_used *)
>                           (uintptr_t)vq->device_addr);
>
> +       vq->vring.last_avail_idx = last_avail_idx;

Does this need to be serialized with the datapath?

E.g in set_vq_state() we do:

spin_lock(&vdpasim->lock);
vrh->last_avail_idx = state->split.avail_index;
spin_unlock(&vdpasim->lock);

Thanks

>         vq->vring.notify = vdpasim_vq_notify;
>  }
>
> --
> 2.31.1
>

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Re: [PATCH 2/2] vringh: fetch used_idx from vring at vringh_init_iotlb

[Jason Wang]

On Thu, Jan 19, 2023 at 12:44 AM Eugenio Pérez <eperezma@redhat.com> wrote:
>
> Starting from an used_idx different than 0 is needed in use cases like
> virtual machine migration.  Not doing so and letting the caller set an
> avail idx different than 0 causes destination device to try to use old
> buffers that source driver already recover and are not available
> anymore.
>
> While callers like vdpa_sim set avail_idx directly it does not set
> used_idx.  Instead of let the caller do the assignment, fetch it from
> the guest at initialization like vhost-kernel do.
>
> To perform the same at vring_kernel_init and vring_user_init is left for
> the future.
>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>  drivers/vhost/vringh.c | 25 +++++++++++++++++++++++--
>  1 file changed, 23 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> index 33eb941fcf15..0eed825197f2 100644
> --- a/drivers/vhost/vringh.c
> +++ b/drivers/vhost/vringh.c
> @@ -1301,6 +1301,17 @@ static inline int putused_iotlb(const struct vringh *vrh,
>         return 0;
>  }
>
> +/**
> + * vringh_update_used_idx - fetch used idx from driver's used split vring
> + * @vrh: The vring.
> + *
> + * Returns -errno or 0.
> + */
> +static inline int vringh_update_used_idx(struct vringh *vrh)
> +{
> +       return getu16_iotlb(vrh, &vrh->last_used_idx, &vrh->vring.used->idx);
> +}
> +
>  /**
>   * vringh_init_iotlb - initialize a vringh for a ring with IOTLB.
>   * @vrh: the vringh to initialize.
> @@ -1319,8 +1330,18 @@ int vringh_init_iotlb(struct vringh *vrh, u64 features,
>                       struct vring_avail *avail,
>                       struct vring_used *used)
>  {

While at this, I wonder if it's better to have a dedicated parameter
for last_avail_idx?

> -       return vringh_init_kern(vrh, features, num, weak_barriers,
> -                               desc, avail, used);
> +       int r = vringh_init_kern(vrh, features, num, weak_barriers, desc,
> +                                avail, used);
> +
> +       if (r != 0)
> +               return r;
> +
> +       /* Consider the ring not initialized */
> +       if ((void *)desc == used)
> +               return 0;

I don't understand when we can get this (actually it should be a bug
of the caller).

Thanks

> +
> +       return vringh_update_used_idx(vrh);
> +
>  }
>  EXPORT_SYMBOL(vringh_init_iotlb);
>
> --
> 2.31.1
>

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Re: [PATCH 0/2] Fix expected set_vq_state behavior on vdpa_sim

[Michael S. Tsirkin]

On Wed, Jan 18, 2023 at 05:43:57PM +0100, Eugenio Pérez wrote:
> The use of set_vq_state is to indicate vdpa device the state of a virtqueue.
> In the case of split, it means the avail_idx.  This is mandatory for use
> cases like live migration.
> 
> However, vdpa_sim reset the vq state at vdpasim_queue_ready since it calls
> vringh_init_iotlb.
> 
> Also, to starting from an used_idx different than 0 is needed in use cases like
> virtual machine migration.  Not doing so and letting the caller set an avail
> idx different than 0 causes destination device to try to use old buffers that
> source driver already recover and are not available anymore.
> 
> This series fixes both problems allowing to migrate to a vdpa_sim_net device.

Jason problems you pointed out are all consmetic do you ack
the patchset? Or expect another revision?

> Eugenio Pérez (2):
>   vdpa_sim: not reset state in vdpasim_queue_ready
>   vringh: fetch used_idx from vring at vringh_init_iotlb
> 
>  drivers/vdpa/vdpa_sim/vdpa_sim.c |  2 ++
>  drivers/vhost/vringh.c           | 25 +++++++++++++++++++++++--
>  2 files changed, 25 insertions(+), 2 deletions(-)
> 
> -- 
> 2.31.1
> 

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Re: [PATCH 1/2] vdpa_sim: not reset state in vdpasim_queue_ready

[Jason Wang]

在 2023/1/19 17:14, Eugenio Perez Martin 写道:
> On Thu, Jan 19, 2023 at 4:16 AM Jason Wang <jasowang@redhat.com> wrote:
>> On Thu, Jan 19, 2023 at 12:44 AM Eugenio Pérez <eperezma@redhat.com> wrote:
>>> vdpasim_queue_ready calls vringh_init_iotlb, which resets split indexes.
>>> But it can be called after setting a ring base with
>>> vdpasim_set_vq_state.
>>>
>>> Fix it by stashing them. They're still resetted in vdpasim_vq_reset.
>>>
>>> This was discovered and tested live migrating the vdpa_sim_net device.
>>>
>>> Fixes: 2c53d0f64c06 ("vdpasim: vDPA device simulator")
>>> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
>>> ---
>>>   drivers/vdpa/vdpa_sim/vdpa_sim.c | 2 ++
>>>   1 file changed, 2 insertions(+)
>>>
>>> diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.c b/drivers/vdpa/vdpa_sim/vdpa_sim.c
>>> index cb88891b44a8..8839232a3fcb 100644
>>> --- a/drivers/vdpa/vdpa_sim/vdpa_sim.c
>>> +++ b/drivers/vdpa/vdpa_sim/vdpa_sim.c
>>> @@ -66,6 +66,7 @@ static void vdpasim_vq_notify(struct vringh *vring)
>>>   static void vdpasim_queue_ready(struct vdpasim *vdpasim, unsigned int idx)
>>>   {
>>>          struct vdpasim_virtqueue *vq = &vdpasim->vqs[idx];
>>> +       uint16_t last_avail_idx = vq->vring.last_avail_idx;
>>>
>>>          vringh_init_iotlb(&vq->vring, vdpasim->features, vq->num, false,
>>>                            (struct vring_desc *)(uintptr_t)vq->desc_addr,
>>> @@ -74,6 +75,7 @@ static void vdpasim_queue_ready(struct vdpasim *vdpasim, unsigned int idx)
>>>                            (struct vring_used *)
>>>                            (uintptr_t)vq->device_addr);
>>>
>>> +       vq->vring.last_avail_idx = last_avail_idx;
>> Does this need to be serialized with the datapath?
>>
>> E.g in set_vq_state() we do:
>>
>> spin_lock(&vdpasim->lock);
>> vrh->last_avail_idx = state->split.avail_index;
>> spin_unlock(&vdpasim->lock);
>>
> vdpasim_queue_ready is called from vdpasim_set_vq_ready, which holds
> these locks.
>
> Maybe it's too much indirection and to embed vdpasim_queue_ready in
> vdpasim_set_vq_ready would be clearer for the future?


Nope, I miss the caller.

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks


>
> Thanks!
>

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Re: [PATCH 2/2] vringh: fetch used_idx from vring at vringh_init_iotlb

[Jason Wang]

On Thu, Jan 19, 2023 at 4:11 PM Eugenio Perez Martin
<eperezma@redhat.com> wrote:
>
> On Thu, Jan 19, 2023 at 4:20 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> > On Thu, Jan 19, 2023 at 12:44 AM Eugenio Pérez <eperezma@redhat.com> wrote:
> > >
> > > Starting from an used_idx different than 0 is needed in use cases like
> > > virtual machine migration.  Not doing so and letting the caller set an
> > > avail idx different than 0 causes destination device to try to use old
> > > buffers that source driver already recover and are not available
> > > anymore.
> > >
> > > While callers like vdpa_sim set avail_idx directly it does not set
> > > used_idx.  Instead of let the caller do the assignment, fetch it from
> > > the guest at initialization like vhost-kernel do.
> > >
> > > To perform the same at vring_kernel_init and vring_user_init is left for
> > > the future.
> > >
> > > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > > ---
> > >  drivers/vhost/vringh.c | 25 +++++++++++++++++++++++--
> > >  1 file changed, 23 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > > index 33eb941fcf15..0eed825197f2 100644
> > > --- a/drivers/vhost/vringh.c
> > > +++ b/drivers/vhost/vringh.c
> > > @@ -1301,6 +1301,17 @@ static inline int putused_iotlb(const struct vringh *vrh,
> > >         return 0;
> > >  }
> > >
> > > +/**
> > > + * vringh_update_used_idx - fetch used idx from driver's used split vring
> > > + * @vrh: The vring.
> > > + *
> > > + * Returns -errno or 0.
> > > + */
> > > +static inline int vringh_update_used_idx(struct vringh *vrh)
> > > +{
> > > +       return getu16_iotlb(vrh, &vrh->last_used_idx, &vrh->vring.used->idx);
> > > +}
> > > +
> > >  /**
> > >   * vringh_init_iotlb - initialize a vringh for a ring with IOTLB.
> > >   * @vrh: the vringh to initialize.
> > > @@ -1319,8 +1330,18 @@ int vringh_init_iotlb(struct vringh *vrh, u64 features,
> > >                       struct vring_avail *avail,
> > >                       struct vring_used *used)
> > >  {
> >
> > While at this, I wonder if it's better to have a dedicated parameter
> > for last_avail_idx?
> >
>
> I also had that thought. To directly assign last_avail_idx is not a
> specially elegant API IMO.
>
> Maybe expose a way to fetch used_idx from device vring and pass
> used_idx as parameter too?

If I was not wrong, we can start from last_avail_idx, for used_idx it
is only needed for inflight descriptors which might require other
APIs?

(All the current vDPA user of vringh is doing in order processing)

>
> > > -       return vringh_init_kern(vrh, features, num, weak_barriers,
> > > -                               desc, avail, used);
> > > +       int r = vringh_init_kern(vrh, features, num, weak_barriers, desc,
> > > +                                avail, used);
> > > +
> > > +       if (r != 0)
> > > +               return r;
> > > +
> > > +       /* Consider the ring not initialized */
> > > +       if ((void *)desc == used)
> > > +               return 0;
> >
> > I don't understand when we can get this (actually it should be a bug
> > of the caller).
> >
>
> You can see it in vdpasim_vq_reset.
>
> Note that to consider desc == 0 to be an uninitialized ring is a bug
> IMO. QEMU considers it that way also, but the standard does not forbid
> any ring to be at address 0. Especially if we use vIOMMU.
>
> So I think the best way to know if we can use the vringh is either
> this way, or provide an explicit "initialized" boolean attribute.
> Maybe a new "bool is_initialized(vrh)" is enough, if we don't want to
> add new attributes.

I wonder if we can avoid this in the simulator level instead of the
vringh (anyhow it only exposes a vringh_init_xxx() helper now).

Thanks

>
> Thanks!
>
> > Thanks
> >
> > > +
> > > +       return vringh_update_used_idx(vrh);
> > > +
> > >  }
> > >  EXPORT_SYMBOL(vringh_init_iotlb);
> > >
> > > --
> > > 2.31.1
> > >
> >
>

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Re: [PATCH 2/2] vringh: fetch used_idx from vring at vringh_init_iotlb

[Jason Wang]

On Tue, Jan 31, 2023 at 12:39 AM Eugenio Perez Martin
<eperezma@redhat.com> wrote:
>
> On Sun, Jan 29, 2023 at 7:01 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> > On Thu, Jan 19, 2023 at 4:11 PM Eugenio Perez Martin
> > <eperezma@redhat.com> wrote:
> > >
> > > On Thu, Jan 19, 2023 at 4:20 AM Jason Wang <jasowang@redhat.com> wrote:
> > > >
> > > > On Thu, Jan 19, 2023 at 12:44 AM Eugenio Pérez <eperezma@redhat.com> wrote:
> > > > >
> > > > > Starting from an used_idx different than 0 is needed in use cases like
> > > > > virtual machine migration.  Not doing so and letting the caller set an
> > > > > avail idx different than 0 causes destination device to try to use old
> > > > > buffers that source driver already recover and are not available
> > > > > anymore.
> > > > >
> > > > > While callers like vdpa_sim set avail_idx directly it does not set
> > > > > used_idx.  Instead of let the caller do the assignment, fetch it from
> > > > > the guest at initialization like vhost-kernel do.
> > > > >
> > > > > To perform the same at vring_kernel_init and vring_user_init is left for
> > > > > the future.
> > > > >
> > > > > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > > > > ---
> > > > >  drivers/vhost/vringh.c | 25 +++++++++++++++++++++++--
> > > > >  1 file changed, 23 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > > > > index 33eb941fcf15..0eed825197f2 100644
> > > > > --- a/drivers/vhost/vringh.c
> > > > > +++ b/drivers/vhost/vringh.c
> > > > > @@ -1301,6 +1301,17 @@ static inline int putused_iotlb(const struct vringh *vrh,
> > > > >         return 0;
> > > > >  }
> > > > >
> > > > > +/**
> > > > > + * vringh_update_used_idx - fetch used idx from driver's used split vring
> > > > > + * @vrh: The vring.
> > > > > + *
> > > > > + * Returns -errno or 0.
> > > > > + */
> > > > > +static inline int vringh_update_used_idx(struct vringh *vrh)
> > > > > +{
> > > > > +       return getu16_iotlb(vrh, &vrh->last_used_idx, &vrh->vring.used->idx);
> > > > > +}
> > > > > +
> > > > >  /**
> > > > >   * vringh_init_iotlb - initialize a vringh for a ring with IOTLB.
> > > > >   * @vrh: the vringh to initialize.
> > > > > @@ -1319,8 +1330,18 @@ int vringh_init_iotlb(struct vringh *vrh, u64 features,
> > > > >                       struct vring_avail *avail,
> > > > >                       struct vring_used *used)
> > > > >  {
> > > >
> > > > While at this, I wonder if it's better to have a dedicated parameter
> > > > for last_avail_idx?
> > > >
> > >
> > > I also had that thought. To directly assign last_avail_idx is not a
> > > specially elegant API IMO.
> > >
> > > Maybe expose a way to fetch used_idx from device vring and pass
> > > used_idx as parameter too?
> >
> > If I was not wrong, we can start from last_avail_idx, for used_idx it
> > is only needed for inflight descriptors which might require other
> > APIs?
> >
> > (All the current vDPA user of vringh is doing in order processing)
> >
>
> That was actually my first attempt and it works equally well for the
> moment, but it diverges from vhost-kernel behavior for little benefit.
>
> To assign both values at set_vring_base mean that if vDPA introduces
> an (hypothetical) VHOST_VDPA_F_INFLIGHT backend feature in the future,
> the initialization process would vary a lot:
> * Without that feature, the used_idx starts with 0, and the avail one
> is 0 or whatever value the user set with vring_set_base.
> * With that feature, the device will read guest's used_idx as
> vhost-kernel? We would enable a new ioctl to set it, or expand
> set_base to include used_idx, effectively diverting from vhost-kernel?

Adding Longpeng who is looking at this.

We can leave this to the caller to decide.

Btw, a question, at which case the device used index does not equal to
the used index in the vring when the device is suspended? I think the
correct way is to flush the pending used indexes before suspending.
Otherwise we need an API to get pending used indices?

>
> To me the wisest option is to move this with vhost-kernel. Maybe we
> need to add a feature bit to know that the hypervisor can trust the
> device will do "the right thing" (VHOST_VDPA_F_FETCH_USED_AT_ENABLE?),
> but we should keep it orthogonal to inflight descriptor migration in
> my opinion.

I think we need to understand if there are any other possible use
cases for setting used idx other than inflight stuff.

>
> Having said that, I'm totally ok to do it otherwise (or to expand the
> patch message if needed).

I tend to do that in another series (not mix with the fixes).

>
> > >
> > > > > -       return vringh_init_kern(vrh, features, num, weak_barriers,
> > > > > -                               desc, avail, used);
> > > > > +       int r = vringh_init_kern(vrh, features, num, weak_barriers, desc,
> > > > > +                                avail, used);
> > > > > +
> > > > > +       if (r != 0)
> > > > > +               return r;
> > > > > +
> > > > > +       /* Consider the ring not initialized */
> > > > > +       if ((void *)desc == used)
> > > > > +               return 0;
> > > >
> > > > I don't understand when we can get this (actually it should be a bug
> > > > of the caller).
> > > >
> > >
> > > You can see it in vdpasim_vq_reset.
> > >
> > > Note that to consider desc == 0 to be an uninitialized ring is a bug
> > > IMO. QEMU considers it that way also, but the standard does not forbid
> > > any ring to be at address 0. Especially if we use vIOMMU.
> > >
> > > So I think the best way to know if we can use the vringh is either
> > > this way, or provide an explicit "initialized" boolean attribute.
> > > Maybe a new "bool is_initialized(vrh)" is enough, if we don't want to
> > > add new attributes.
> >
> > I wonder if we can avoid this in the simulator level instead of the
> > vringh (anyhow it only exposes a vringh_init_xxx() helper now).
> >
>
> In my opinion that is a mistake if other drivers will use it to
> implement the emulated control virtqueue. And it requires more
> changes. But it is doable for sure.

The problem is, there's no reset API in vringh, that's why you need to
do if ((void *)desc == used) which depends on behaviour of the vringh
user.

So I think we should either:

1) move that check in vdpa_sim (since it's not guaranteed that all the
vringh users will make desc equal to used during reset)

or

2) introduce a vringh_reset_xxx()

1) seems a good step for -stable.

Thanks

>
> Thanks!
>

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Re: [PATCH 2/2] vringh: fetch used_idx from vring at vringh_init_iotlb

[Michael S. Tsirkin]

On Wed, Jan 18, 2023 at 05:43:59PM +0100, Eugenio Pérez wrote:
> Starting from an used_idx different than 0 is needed in use cases like
> virtual machine migration.  Not doing so and letting the caller set an
> avail idx different than 0 causes destination device to try to use old
> buffers that source driver already recover and are not available
> anymore.
> 
> While callers like vdpa_sim set avail_idx directly it does not set
> used_idx.  Instead of let the caller do the assignment, fetch it from
> the guest at initialization like vhost-kernel do.
> 
> To perform the same at vring_kernel_init and vring_user_init is left for
> the future.
> 
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>


So I applied 1/2 and dropped 2/2 for now, right?

> ---
>  drivers/vhost/vringh.c | 25 +++++++++++++++++++++++--
>  1 file changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> index 33eb941fcf15..0eed825197f2 100644
> --- a/drivers/vhost/vringh.c
> +++ b/drivers/vhost/vringh.c
> @@ -1301,6 +1301,17 @@ static inline int putused_iotlb(const struct vringh *vrh,
>  	return 0;
>  }
>  
> +/**
> + * vringh_update_used_idx - fetch used idx from driver's used split vring
> + * @vrh: The vring.
> + *
> + * Returns -errno or 0.
> + */
> +static inline int vringh_update_used_idx(struct vringh *vrh)
> +{
> +	return getu16_iotlb(vrh, &vrh->last_used_idx, &vrh->vring.used->idx);
> +}
> +
>  /**
>   * vringh_init_iotlb - initialize a vringh for a ring with IOTLB.
>   * @vrh: the vringh to initialize.
> @@ -1319,8 +1330,18 @@ int vringh_init_iotlb(struct vringh *vrh, u64 features,
>  		      struct vring_avail *avail,
>  		      struct vring_used *used)
>  {
> -	return vringh_init_kern(vrh, features, num, weak_barriers,
> -				desc, avail, used);
> +	int r = vringh_init_kern(vrh, features, num, weak_barriers, desc,
> +				 avail, used);
> +
> +	if (r != 0)
> +		return r;
> +
> +	/* Consider the ring not initialized */
> +	if ((void *)desc == used)
> +		return 0;
> +
> +	return vringh_update_used_idx(vrh);
> +
>  }
>  EXPORT_SYMBOL(vringh_init_iotlb);
>  
> -- 
> 2.31.1

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Re: [PATCH 2/2] vringh: fetch used_idx from vring at vringh_init_iotlb

[Michael S. Tsirkin]

On Tue, Jan 31, 2023 at 08:58:37AM +0100, Eugenio Perez Martin wrote:
> On Tue, Jan 31, 2023 at 4:16 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> > On Tue, Jan 31, 2023 at 12:39 AM Eugenio Perez Martin
> > <eperezma@redhat.com> wrote:
> > >
> > > On Sun, Jan 29, 2023 at 7:01 AM Jason Wang <jasowang@redhat.com> wrote:
> > > >
> > > > On Thu, Jan 19, 2023 at 4:11 PM Eugenio Perez Martin
> > > > <eperezma@redhat.com> wrote:
> > > > >
> > > > > On Thu, Jan 19, 2023 at 4:20 AM Jason Wang <jasowang@redhat.com> wrote:
> > > > > >
> > > > > > On Thu, Jan 19, 2023 at 12:44 AM Eugenio Pérez <eperezma@redhat.com> wrote:
> > > > > > >
> > > > > > > Starting from an used_idx different than 0 is needed in use cases like
> > > > > > > virtual machine migration.  Not doing so and letting the caller set an
> > > > > > > avail idx different than 0 causes destination device to try to use old
> > > > > > > buffers that source driver already recover and are not available
> > > > > > > anymore.
> > > > > > >
> > > > > > > While callers like vdpa_sim set avail_idx directly it does not set
> > > > > > > used_idx.  Instead of let the caller do the assignment, fetch it from
> > > > > > > the guest at initialization like vhost-kernel do.
> > > > > > >
> > > > > > > To perform the same at vring_kernel_init and vring_user_init is left for
> > > > > > > the future.
> > > > > > >
> > > > > > > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > > > > > > ---
> > > > > > >  drivers/vhost/vringh.c | 25 +++++++++++++++++++++++--
> > > > > > >  1 file changed, 23 insertions(+), 2 deletions(-)
> > > > > > >
> > > > > > > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > > > > > > index 33eb941fcf15..0eed825197f2 100644
> > > > > > > --- a/drivers/vhost/vringh.c
> > > > > > > +++ b/drivers/vhost/vringh.c
> > > > > > > @@ -1301,6 +1301,17 @@ static inline int putused_iotlb(const struct vringh *vrh,
> > > > > > >         return 0;
> > > > > > >  }
> > > > > > >
> > > > > > > +/**
> > > > > > > + * vringh_update_used_idx - fetch used idx from driver's used split vring
> > > > > > > + * @vrh: The vring.
> > > > > > > + *
> > > > > > > + * Returns -errno or 0.
> > > > > > > + */
> > > > > > > +static inline int vringh_update_used_idx(struct vringh *vrh)
> > > > > > > +{
> > > > > > > +       return getu16_iotlb(vrh, &vrh->last_used_idx, &vrh->vring.used->idx);
> > > > > > > +}
> > > > > > > +
> > > > > > >  /**
> > > > > > >   * vringh_init_iotlb - initialize a vringh for a ring with IOTLB.
> > > > > > >   * @vrh: the vringh to initialize.
> > > > > > > @@ -1319,8 +1330,18 @@ int vringh_init_iotlb(struct vringh *vrh, u64 features,
> > > > > > >                       struct vring_avail *avail,
> > > > > > >                       struct vring_used *used)
> > > > > > >  {
> > > > > >
> > > > > > While at this, I wonder if it's better to have a dedicated parameter
> > > > > > for last_avail_idx?
> > > > > >
> > > > >
> > > > > I also had that thought. To directly assign last_avail_idx is not a
> > > > > specially elegant API IMO.
> > > > >
> > > > > Maybe expose a way to fetch used_idx from device vring and pass
> > > > > used_idx as parameter too?
> > > >
> > > > If I was not wrong, we can start from last_avail_idx, for used_idx it
> > > > is only needed for inflight descriptors which might require other
> > > > APIs?
> > > >
> > > > (All the current vDPA user of vringh is doing in order processing)
> > > >
> > >
> > > That was actually my first attempt and it works equally well for the
> > > moment, but it diverges from vhost-kernel behavior for little benefit.
> > >
> > > To assign both values at set_vring_base mean that if vDPA introduces
> > > an (hypothetical) VHOST_VDPA_F_INFLIGHT backend feature in the future,
> > > the initialization process would vary a lot:
> > > * Without that feature, the used_idx starts with 0, and the avail one
> > > is 0 or whatever value the user set with vring_set_base.
> > > * With that feature, the device will read guest's used_idx as
> > > vhost-kernel? We would enable a new ioctl to set it, or expand
> > > set_base to include used_idx, effectively diverting from vhost-kernel?
> >
> > Adding Longpeng who is looking at this.
> >
> 
> Sorry, I'll CC longpeng2@huawei.com in future series like this one.
> 
> > We can leave this to the caller to decide.
> >
> > Btw, a question, at which case the device used index does not equal to
> > the used index in the vring when the device is suspended? I think the
> > correct way is to flush the pending used indexes before suspending.
> > Otherwise we need an API to get pending used indices?
> >
> > >
> > > To me the wisest option is to move this with vhost-kernel. Maybe we
> > > need to add a feature bit to know that the hypervisor can trust the
> > > device will do "the right thing" (VHOST_VDPA_F_FETCH_USED_AT_ENABLE?),
> > > but we should keep it orthogonal to inflight descriptor migration in
> > > my opinion.
> >
> > I think we need to understand if there are any other possible use
> > cases for setting used idx other than inflight stuff.
> >
> 
> Answering this and the previous comment, I cannot think in any case
> outside of inflight migration. I'm just trying to avoid different
> behavior between vhost-kernel and vhost-vdpa, and to make features as
> orthogonal as possible.
> 
> > >
> > > Having said that, I'm totally ok to do it otherwise (or to expand the
> > > patch message if needed).
> >
> > I tend to do that in another series (not mix with the fixes).
> >
> > >
> > > > >
> > > > > > > -       return vringh_init_kern(vrh, features, num, weak_barriers,
> > > > > > > -                               desc, avail, used);
> > > > > > > +       int r = vringh_init_kern(vrh, features, num, weak_barriers, desc,
> > > > > > > +                                avail, used);
> > > > > > > +
> > > > > > > +       if (r != 0)
> > > > > > > +               return r;
> > > > > > > +
> > > > > > > +       /* Consider the ring not initialized */
> > > > > > > +       if ((void *)desc == used)
> > > > > > > +               return 0;
> > > > > >
> > > > > > I don't understand when we can get this (actually it should be a bug
> > > > > > of the caller).
> > > > > >
> > > > >
> > > > > You can see it in vdpasim_vq_reset.
> > > > >
> > > > > Note that to consider desc == 0 to be an uninitialized ring is a bug
> > > > > IMO. QEMU considers it that way also, but the standard does not forbid
> > > > > any ring to be at address 0. Especially if we use vIOMMU.
> > > > >
> > > > > So I think the best way to know if we can use the vringh is either
> > > > > this way, or provide an explicit "initialized" boolean attribute.
> > > > > Maybe a new "bool is_initialized(vrh)" is enough, if we don't want to
> > > > > add new attributes.
> > > >
> > > > I wonder if we can avoid this in the simulator level instead of the
> > > > vringh (anyhow it only exposes a vringh_init_xxx() helper now).
> > > >
> > >
> > > In my opinion that is a mistake if other drivers will use it to
> > > implement the emulated control virtqueue. And it requires more
> > > changes. But it is doable for sure.
> >
> > The problem is, there's no reset API in vringh, that's why you need to
> > do if ((void *)desc == used) which depends on behaviour of the vringh
> > user.
> >
> 
> That's a very good point indeed.
> 
> > So I think we should either:
> >
> > 1) move that check in vdpa_sim (since it's not guaranteed that all the
> > vringh users will make desc equal to used during reset)
> >
> > or
> >
> > 2) introduce a vringh_reset_xxx()
> >
> > 1) seems a good step for -stable.
> >
> 
> We can go to 1 for sure. So let's set last_used_idx at
> vdpasim_set_vq_state, same value as last_avail_idx, and stash it at
> vdpasim_queue_ready.
> 
> Do I need to resend the previous patch in this series?
> 
> Do we need to offer a new feature flag indicating we will set used_idx
> with avail_idx?
> 
> Thanks!

Jason did you forget to answer or did I miss it?

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