Re: [RFC PATCH 07/12] virtio/s390: use DMA memory for ccw I/O

[Halil Pasic <pasic@linux.ibm.com>]

On Wed, 10 Apr 2019 10:42:51 +0200
Cornelia Huck <cohuck@redhat.com> wrote:

> On Fri,  5 Apr 2019 01:16:17 +0200
> Halil Pasic <pasic@linux.ibm.com> wrote:
> 
> > Before virtio-ccw could get away with not using DMA API for the pieces of
> > memory it does ccw I/O with. With protected virtualization this has to
> > change, since the hypervisor needs to read and sometimes also write these
> > pieces of memory.
> > 
> > Let us make sure all ccw I/O is done through shared memory.
> > 
> > Note: The control blocks of I/O instructions do not need to be shared.
> > These are marshalled by the ultravisor.
> 
> Ok, so direct parameters of I/O instructions are handled by the
> ultravisor?
> 

Yes.

> > 
> > Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> > ---
> >  drivers/s390/virtio/virtio_ccw.c | 177 +++++++++++++++++++++++----------------
> >  1 file changed, 107 insertions(+), 70 deletions(-)
> > 
> (...)
> > @@ -167,6 +170,28 @@ static struct virtio_ccw_device *to_vc_device(struct virtio_device *vdev)
> >  	return container_of(vdev, struct virtio_ccw_device, vdev);
> >  }
> >  
> > +#define vc_dma_decl_struct(type, field) \
> > +	dma_addr_t field ## _dma_addr;  \
> > +	struct type *field
> > +
> > +static inline void *__vc_dma_alloc(struct virtio_device *vdev, size_t size,
> > +				   dma_addr_t *dma_handle)
> > +{
> > +	return dma_alloc_coherent(vdev->dev.parent, size, dma_handle,
> > +				  GFP_DMA | GFP_KERNEL | __GFP_ZERO);
> > +}
> > +
> > +static inline void __vc_dma_free(struct virtio_device *vdev, size_t size,
> > +				 void *cpu_addr, dma_addr_t dma_handle)
> > +{
> > +	dma_free_coherent(vdev->dev.parent, size, cpu_addr, dma_handle);
> > +}
> > +
> > +#define vc_dma_alloc_struct(vdev, ptr) \
> > +	({ ptr = __vc_dma_alloc(vdev, (sizeof(*(ptr))), &(ptr ## _dma_addr)); })
> > +#define vc_dma_free_struct(vdev, ptr) \
> > +	__vc_dma_free(vdev, sizeof(*(ptr)), (ptr), (ptr ## _dma_addr))
> 
> Not sure I'm a fan of those wrappers... I think they actually hurt
> readability of the code.
> 

By wrappers you mean just the macros or also the inline functions?

If we agree to go with the cio DMA pool instead of using DMA API
facilities for allocation (dma_alloc_coherent or maybe a per ccw-device
dma_pool) I think I could just use cio_dma_zalloc() directly if you like.

I was quite insecure about how this gen_pool idea is going to be received
here. That's why I decided to keep the dma_alloc_coherent() version in
for the RFC.

If you prefer I can squash patches #7 #9 #10 and #11 together and
pull #8 forward. Would you prefer that?


> > +
> >  static void drop_airq_indicator(struct virtqueue *vq, struct airq_info *info)
> >  {
> >  	unsigned long i, flags;
> > @@ -322,12 +347,12 @@ static void virtio_ccw_drop_indicator(struct virtio_ccw_device *vcdev,
> >  {
> >  	int ret;
> >  	unsigned long *indicatorp = NULL;
> > -	struct virtio_thinint_area *thinint_area = NULL;
> > +	vc_dma_decl_struct(virtio_thinint_area, thinint_area) = NULL;
> > +	dma_addr_t indicatorp_dma_addr;
> >  	struct airq_info *airq_info = vcdev->airq_info;
> >  
> >  	if (vcdev->is_thinint) {
> > -		thinint_area = kzalloc(sizeof(*thinint_area),
> > -				       GFP_DMA | GFP_KERNEL);
> > +		vc_dma_alloc_struct(&vcdev->vdev, thinint_area);
> >  		if (!thinint_area)
> >  			return;
> >  		thinint_area->summary_indicator =
> > @@ -338,8 +363,9 @@ static void virtio_ccw_drop_indicator(struct virtio_ccw_device *vcdev,
> >  		ccw->cda = (__u32)(unsigned long) thinint_area;
> >  	} else {
> >  		/* payload is the address of the indicators */
> > -		indicatorp = kmalloc(sizeof(&vcdev->indicators),
> > -				     GFP_DMA | GFP_KERNEL);
> > +		indicatorp = __vc_dma_alloc(&vcdev->vdev,
> > +					    sizeof(&vcdev->indicators),
> > +					    &indicatorp_dma_addr);
> >  		if (!indicatorp)
> >  			return;
> >  		*indicatorp = 0;
> > @@ -359,8 +385,10 @@ static void virtio_ccw_drop_indicator(struct virtio_ccw_device *vcdev,
> >  			 "Failed to deregister indicators (%d)\n", ret);
> >  	else if (vcdev->is_thinint)
> >  		virtio_ccw_drop_indicators(vcdev);
> > -	kfree(indicatorp);
> > -	kfree(thinint_area);
> > +	if (indicatorp)
> > +		__vc_dma_free(&vcdev->vdev, sizeof(&vcdev->indicators),
> > +			       indicatorp, indicatorp_dma_addr);
> > +	vc_dma_free_struct(&vcdev->vdev, thinint_area);
> 
> Don't you need to check for !NULL here as well?

Good catch! 

I could take care of it in __vc_dma_free().

void cio_dma_free(void *cpu_addr, size_t size) {
 +	if (!cpu_addr)
 +		return;

also seems to me like a good idea right now.

> 
> >  }
> >  
> >  static inline long __do_kvm_notify(struct subchannel_id schid,
> (...)
> > @@ -1280,7 +1318,6 @@ static int virtio_ccw_online(struct ccw_device *cdev)
> >  
> >  	vcdev->is_thinint = virtio_ccw_use_airq; /* at least try */
> >  
> > -	vcdev->vdev.dev.parent = &cdev->dev;
> 
> Hm?
> 
> (You added a line like that in a previous patch; should it simply have
> been a movement instead? Or am I misremembering?)

Right the move was supposed to take place in patch #2. Not sure how
I ended up with this. Maybe a messed up rebase.

> 
> >  	vcdev->vdev.dev.release = virtio_ccw_release_dev;
> >  	vcdev->vdev.config = &virtio_ccw_config_ops;
> >  	vcdev->cdev = cdev;
>