Re: [PATCH 06/10] s390/cio: add basic protected virtualization support

Sebastian Ott
2019-05-09T08:52:45Z

On Wed, 8 May 2019, Halil Pasic wrote:
> On Wed, 8 May 2019 15:46:42 +0200 (CEST)
> Sebastian Ott <sebott@linux.ibm.com> wrote:
> > On Fri, 26 Apr 2019, Halil Pasic wrote:
> > > static struct ccw_device * io_subchannel_allocate_dev(struct subchannel *sch)
> > > {
> > [..]
> > > + cdev->private = kzalloc(sizeof(struct ccw_device_private),
> > > + GFP_KERNEL | GFP_DMA);
> >
> > Do we still need GFP_DMA here (since we now have cdev->private->dma_area)?
> >
>
> We probably do not. I kept it GFP_DMA to keep changes to the
> minimum. Should changing this in your opinion be a part of this patch?

This can be changed on top.

> > > @@ -1062,6 +1082,14 @@ static int io_subchannel_probe(struct subchannel *sch)
> > > if (!io_priv)
> > > goto out_schedule;
> > >
> > > + io_priv->dma_area = dma_alloc_coherent(&sch->dev,
> > > + sizeof(*io_priv->dma_area),
> > > + &io_priv->dma_area_dma, GFP_KERNEL);
> >
> > This needs GFP_DMA.
>
> Christoph already answered this one. Thanks Christoph!

Yes, I'm still struggling to grasp the whole channel IO + DMA API +
protected virtualization thing..

>
> > You use a genpool for ccw_private->dma and not for iopriv->dma - looks
> > kinda inconsistent.
> >
>
> Please have a look at patch #9. A virtio-ccw device uses the genpool of
> it's ccw device (the pool from which ccw_private->dma is allicated) for
> the ccw stuff it needs to do. AFAICT for a subchannel device and its API
> all the DMA memory we need is iopriv->dma. So my thought was
> constructing a genpool for that would be an overkill.
>
> Are you comfortable with this answer, or should we change something?

Nope, I'm good with that one - ccw_private->dma has multiple users that
all fit into one page.