Re: [PATCH v6 18/19] swiotlb-xen: introduce a rbtree to track physto bus mappings

[Stefano Stabellini]

On Mon, 30 Sep 2013, Konrad Rzeszutek Wilk wrote:
> On Fri, Sep 27, 2013 at 05:10:06PM +0100, Stefano Stabellini wrote:
> > Introduce a second red-back tree to track phys to bus mappings created after
> > the initialization of the swiotlb buffer.
> 
> Could you explain the use case a bit more please?
> 
> As in:
>  a) why is this needed
>  b) why are we using the rb tree instead of something else (say FIFO queue)
>  c) how long are these in usage?
>  d) do we need locking now?

Given that I was rebuilding a P2M for autotranslate guests in
swiotlb-xen, making the code much harder to read, I decided to go the
alternative route of providing a real P2M to ARM guest.

The next swiotlb series will introduce arch/arm/xen/p2m.c, moving the
two rbtrees there. As a consequence we can start using swiotlb-xen on
ARM with minimal changes. The whole thing looks much nicer now.



> > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > ---
> >  drivers/xen/swiotlb-xen.c |   99 +++++++++++++++++++++++++++++++++++++-------
> >  1 files changed, 83 insertions(+), 16 deletions(-)
> > 
> > diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> > index 3011736..022bcaf 100644
> > --- a/drivers/xen/swiotlb-xen.c
> > +++ b/drivers/xen/swiotlb-xen.c
> > @@ -79,7 +79,8 @@ struct xen_dma_info {
> >  	dma_addr_t dma_addr;
> >  	phys_addr_t phys_addr;
> >  	size_t size;
> > -	struct rb_node rbnode;
> > +	struct rb_node rbnode_dma;
> > +	struct rb_node rbnode_phys;
> >  };
> >  
> >  /*
> > @@ -96,8 +97,13 @@ static struct xen_dma_info *xen_dma_seg;
> >   * mappings.
> >   */
> >  static struct rb_root bus_to_phys = RB_ROOT;
> > +/*
> > + * This tree keeps track of physical address to bus address
> > + * mappings apart from the ones belonging to the initial swiotlb buffer.
> > + */
> > +static struct rb_root phys_to_bus = RB_ROOT;
> >  
> > -static int xen_dma_add_entry(struct xen_dma_info *new)
> > +static int xen_dma_add_entry_bus(struct xen_dma_info *new)
> >  {
> >  	struct rb_node **link = &bus_to_phys.rb_node;
> >  	struct rb_node *parent = NULL;
> > @@ -106,7 +112,7 @@ static int xen_dma_add_entry(struct xen_dma_info *new)
> >  
> >  	while (*link) {
> >  		parent = *link;
> > -		entry = rb_entry(parent, struct xen_dma_info, rbnode);
> > +		entry = rb_entry(parent, struct xen_dma_info, rbnode_dma);
> >  
> >  		if (new->dma_addr == entry->dma_addr)
> >  			goto err_out;
> > @@ -118,8 +124,41 @@ static int xen_dma_add_entry(struct xen_dma_info *new)
> >  		else
> >  			link = &(*link)->rb_right;
> >  	}
> > -	rb_link_node(&new->rbnode, parent, link);
> > -	rb_insert_color(&new->rbnode, &bus_to_phys);
> > +	rb_link_node(&new->rbnode_dma, parent, link);
> > +	rb_insert_color(&new->rbnode_dma, &bus_to_phys);
> > +	goto out;
> > +
> > +err_out:
> > +	rc = -EINVAL;
> > +	pr_warn("%s: cannot add phys=%pa -> dma=%pa: phys=%pa -> dma=%pa already exists\n",
> > +			__func__, &new->phys_addr, &new->dma_addr, &entry->phys_addr, &entry->dma_addr);
> > +out:
> > +	return rc;
> > +}
> > +
> > +static int xen_dma_add_entry_phys(struct xen_dma_info *new)
> > +{
> > +	struct rb_node **link = &phys_to_bus.rb_node;
> > +	struct rb_node *parent = NULL;
> > +	struct xen_dma_info *entry;
> > +	int rc = 0;
> > +
> > +	while (*link) {
> > +		parent = *link;
> > +		entry = rb_entry(parent, struct xen_dma_info, rbnode_phys);
> > +
> > +		if (new->dma_addr == entry->dma_addr)
> > +			goto err_out;
> > +		if (new->phys_addr == entry->phys_addr)
> > +			goto err_out;
> > +
> > +		if (new->phys_addr < entry->phys_addr)
> > +			link = &(*link)->rb_left;
> > +		else
> > +			link = &(*link)->rb_right;
> > +	}
> > +	rb_link_node(&new->rbnode_phys, parent, link);
> > +	rb_insert_color(&new->rbnode_phys, &phys_to_bus);
> >  	goto out;
> >  
> >  err_out:
> > @@ -130,13 +169,22 @@ out:
> >  	return rc;
> >  }
> >  
> > +static int xen_dma_add_entry(struct xen_dma_info *new)
> > +{
> > +	int rc;
> > +	if ((rc = xen_dma_add_entry_bus(new) < 0) ||
> > +		(rc = xen_dma_add_entry_phys(new) < 0))
> 
> Please don't do that. Just do
> 
> 	rc = xen_dma_add_entry(bus);
> 	if (rc)
> 		return rc;
> 	rc = xen_dma_add_entry_phys(new);
> 	if (rc) {
> 		// unwind it somehow? <<== This is important :-)
> 	}
> 	return rc;
> 
> 
> > +		return rc;
> > +	return 0;
> > +}
> > +
> >  static struct xen_dma_info *xen_get_dma_info_from_dma(dma_addr_t dma_addr)
> >  {
> >  	struct rb_node *n = bus_to_phys.rb_node;
> >  	struct xen_dma_info *entry;
> >  
> >  	while (n) {
> > -		entry = rb_entry(n, struct xen_dma_info, rbnode);
> > +		entry = rb_entry(n, struct xen_dma_info, rbnode_dma);
> >  		if (entry->dma_addr <= dma_addr &&
> >  				entry->dma_addr + entry->size > dma_addr) {
> >  			return entry;
> > @@ -150,11 +198,30 @@ static struct xen_dma_info *xen_get_dma_info_from_dma(dma_addr_t dma_addr)
> >  	return NULL;
> >  }
> >  
> > -static dma_addr_t xen_phys_to_bus(phys_addr_t paddr)
> > +static struct xen_dma_info *xen_get_dma_info_from_phys(phys_addr_t phys)
> >  {
> > -	int nr_seg;
> > -	unsigned long offset;
> > -	char *vaddr;
> > +	struct rb_node *n = phys_to_bus.rb_node;
> > +	struct xen_dma_info *entry;
> > +
> > +	while (n) {
> > +		entry = rb_entry(n, struct xen_dma_info, rbnode_phys);
> > +		if (entry->phys_addr <= phys &&
> > +				entry->phys_addr + entry->size > phys) {
> > +			return entry;
> > +		}
> > +		if (phys < entry->phys_addr)
> > +			n = n->rb_left;
> > +		else
> > +			n = n->rb_right;
> > +	}
> > +
> > +	return NULL;
> > +}
> > +
> > +/* Only looks into the initial buffer allocation in case of
> > + * XENFEAT_auto_translated_physmap guests. */
> > +static dma_addr_t xen_phys_to_bus_quick(phys_addr_t paddr) { int nr_seg;
> > +	unsigned long offset; char *vaddr;
> >  
> >  	if (!xen_feature(XENFEAT_auto_translated_physmap))
> >  		return phys_to_machine(XPADDR(paddr)).maddr;
> > @@ -184,7 +251,7 @@ static phys_addr_t xen_bus_to_phys(dma_addr_t baddr)
> >  
> >  static dma_addr_t xen_virt_to_bus(void *address)
> >  {
> > -	return xen_phys_to_bus(virt_to_phys(address));
> > +	return xen_phys_to_bus_quick(virt_to_phys(address));
> >  }
> >  
> >  static int check_pages_physically_contiguous(unsigned long pfn,
> > @@ -424,7 +491,7 @@ xen_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
> >  	 * Do not use virt_to_phys(ret) because on ARM it doesn't correspond
> >  	 * to *dma_handle. */
> >  	phys = *dma_handle;
> > -	dev_addr = xen_phys_to_bus(phys);
> > +	dev_addr = xen_phys_to_bus_quick(phys);
> >  	if (!xen_feature(XENFEAT_auto_translated_physmap) &&
> >  	    ((dev_addr + size - 1 <= dma_mask)) &&
> >  	    !range_straddles_page_boundary(phys, size))
> > @@ -503,7 +570,7 @@ dma_addr_t xen_swiotlb_map_page(struct device *dev, struct page *page,
> >  				struct dma_attrs *attrs)
> >  {
> >  	phys_addr_t map, phys = page_to_phys(page) + offset;
> > -	dma_addr_t dev_addr = xen_phys_to_bus(phys);
> > +	dma_addr_t dev_addr = xen_phys_to_bus_quick(phys);
> >  
> >  	BUG_ON(dir == DMA_NONE);
> >  	/*
> > @@ -527,7 +594,7 @@ dma_addr_t xen_swiotlb_map_page(struct device *dev, struct page *page,
> >  	if (map == SWIOTLB_MAP_ERROR)
> >  		return DMA_ERROR_CODE;
> >  
> > -	dev_addr = xen_phys_to_bus(map);
> > +	dev_addr = xen_phys_to_bus_quick(map);
> >  
> >  	/*
> >  	 * Ensure that the address returned is DMA'ble
> > @@ -656,7 +723,7 @@ xen_swiotlb_map_sg_attrs(struct device *hwdev, struct scatterlist *sgl,
> >  
> >  	for_each_sg(sgl, sg, nelems, i) {
> >  		phys_addr_t paddr = sg_phys(sg);
> > -		dma_addr_t dev_addr = xen_phys_to_bus(paddr);
> > +		dma_addr_t dev_addr = xen_phys_to_bus_quick(paddr);
> >  
> >  		if (swiotlb_force ||
> >  		    xen_feature(XENFEAT_auto_translated_physmap) ||
> > @@ -682,7 +749,7 @@ xen_swiotlb_map_sg_attrs(struct device *hwdev, struct scatterlist *sgl,
> >  				sg_dma_len(sgl) = 0;
> >  				return DMA_ERROR_CODE;
> >  			}
> > -			sg->dma_address = xen_phys_to_bus(map);
> > +			sg->dma_address = xen_phys_to_bus_quick(map);
> >  		} else
> >  			sg->dma_address = dev_addr;
> >  		sg_dma_len(sg) = sg->length;
> > -- 
> > 1.7.2.5
> > 
> 
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