Re: [PATCH 04/12] pci-p2p: Clear ACS P2P flags for all client devices

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 4 Jan 2018 15:57:21 -0600
Bjorn Helgaas <helgaas@kernel.org> wrote:

> [+cc Alex]
> 
> On Thu, Jan 04, 2018 at 12:01:29PM -0700, Logan Gunthorpe wrote:
> > When the ACS P2P flags are set in the downstream port of the switch,
> > any P2P TLPs will be sent back to the root complex. The whole point of
> > the P2P work is to have TLPs avoid the RC seeing it may not support
> > P2P transactions or, at best, it will perform poorly. So we clear these  
> 
> "It will perform poorly" seems like an empirical statement about the
> hardware you've seen, not something that's driven by the spec.  So I'm
> not sure it adds information that will be useful long-term.
> 
> > flags for all devices involved in transactions with the p2pmem.  
> 
> I'm not sure how this coordinates with other uses of ACS, e.g., VFIO.
> I think that should be addressed here somehow.

Yep, flipping these ACS bits invalidates any IOMMU groups that depend
on the isolation of that downstream port and I suspect also any peers
within the same PCI slot of that port and their downstream devices.  The
entire sub-hierarchy grouping needs to be re-evaluated.  This
potentially affects running devices that depend on that isolation, so
I'm not sure how that happens dynamically.  A boot option might be
easier.  Thanks,

Alex

> > A count of the number of requests to disable the flags is maintained.
> > When the count transitions from 1 to 0, the old flags are restored.
> > 
> > Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> > ---
> >  drivers/pci/p2p.c   | 144 ++++++++++++++++++++++++++++++++++++++++++++++++++--
> >  include/linux/pci.h |   2 +
> >  2 files changed, 143 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/pci/p2p.c b/drivers/pci/p2p.c
> > index 87cec87b02e3..617adaa905d2 100644
> > --- a/drivers/pci/p2p.c
> > +++ b/drivers/pci/p2p.c
> > @@ -227,6 +227,89 @@ static struct pci_dev *find_parent_pci_dev(struct device *dev)
> >  }
> >  
> >  /*
> > + * The ACS flags for P2P Request Redirect and P2P Completion Redirect need
> > + * to be disabled in the downstream port of each device in order for
> > + * the TLPs to not be forwarded up to the RC.
> > + */
> > +#define PCI_P2P_ACS_FLAGS (PCI_ACS_RR | PCI_ACS_CR)
> > +
> > +static int pci_p2pmem_disable_acs(struct pci_dev *pdev)
> > +{
> > +	int pos;
> > +	u16 ctrl;
> > +	struct pci_dev *downstream;
> > +
> > +	downstream = pci_dev_get(pci_upstream_bridge(pdev));
> > +	if (!downstream) {
> > +		dev_err(&pdev->dev, "could not find downstream port\n");
> > +		return -ENODEV;
> > +	}
> > +
> > +	device_lock(&downstream->dev);
> > +	if (downstream->p2p_acs_requests++)
> > +		goto unlock_and_return;
> > +
> > +	pos = pci_find_ext_capability(downstream, PCI_EXT_CAP_ID_ACS);
> > +	if (!pos)
> > +		goto unlock_and_return;
> > +
> > +	pci_read_config_word(downstream, pos + PCI_ACS_CTRL, &ctrl);
> > +
> > +	downstream->p2p_old_acs_flags = ctrl & PCI_P2P_ACS_FLAGS;
> > +
> > +	if (downstream->p2p_old_acs_flags)
> > +		dev_info(&pdev->dev, "disabling p2p acs flags: %x\n", ctrl);
> > +
> > +	ctrl &= ~PCI_P2P_ACS_FLAGS;
> > +
> > +	pci_write_config_word(downstream, pos + PCI_ACS_CTRL, ctrl);
> > +
> > +unlock_and_return:
> > +	device_unlock(&downstream->dev);
> > +	pci_dev_put(downstream);
> > +	return 0;
> > +}
> > +
> > +static int pci_p2pmem_reset_acs(struct pci_dev *pdev)
> > +{
> > +	int pos;
> > +	u16 ctrl;
> > +	struct pci_dev *downstream;
> > +
> > +	downstream = pci_dev_get(pci_upstream_bridge(pdev));
> > +	if (!downstream)
> > +		return -ENODEV;
> > +
> > +	device_lock(&downstream->dev);
> > +
> > +	/* Only actually reset the flags on a 1->0 transition */
> > +	if (!downstream->p2p_acs_requests)
> > +		goto unlock_and_return;
> > +
> > +	if (--downstream->p2p_acs_requests)
> > +		goto unlock_and_return;
> > +
> > +	pos = pci_find_ext_capability(downstream, PCI_EXT_CAP_ID_ACS);
> > +	if (!pos)
> > +		goto unlock_and_return;
> > +
> > +	pci_read_config_word(downstream, pos + PCI_ACS_CTRL, &ctrl);
> > +
> > +	ctrl &= ~PCI_P2P_ACS_FLAGS;
> > +	ctrl |= downstream->p2p_old_acs_flags;
> > +
> > +	if (downstream->p2p_old_acs_flags)
> > +		dev_info(&pdev->dev, "resetting p2p acs flags: %x\n", ctrl);
> > +
> > +	pci_write_config_word(downstream, pos + PCI_ACS_CTRL, ctrl);
> > +
> > +unlock_and_return:
> > +	device_unlock(&downstream->dev);
> > +	pci_dev_put(downstream);
> > +	return 0;
> > +}
> > +
> > +/*
> >   * If a device is behind a switch, we try to find the upstream bridge
> >   * port of the switch. This requires two calls to pci_upstream_bridge:
> >   * one for the upstream port on the switch, one on the upstream port
> > @@ -340,6 +423,10 @@ int pci_p2pmem_add_client(struct list_head *head, struct device *dev)
> >  			ret = -EXDEV;
> >  			goto put_client;
> >  		}
> > +
> > +		ret = pci_p2pmem_disable_acs(client);
> > +		if (!ret)
> > +			goto put_client;
> >  	}
> >  
> >  	new_item = kzalloc(sizeof(*new_item), GFP_KERNEL);
> > @@ -363,6 +450,9 @@ EXPORT_SYMBOL_GPL(pci_p2pmem_add_client);
> >  
> >  static void pci_p2pmem_client_free(struct pci_p2pmem_client *item)
> >  {
> > +	if (item->p2pmem)
> > +		pci_p2pmem_reset_acs(item->client);
> > +
> >  	list_del(&item->list);
> >  	pci_dev_put(item->client);
> >  	pci_dev_put(item->p2pmem);
> > @@ -383,6 +473,7 @@ void pci_p2pmem_remove_client(struct list_head *head, struct device *dev)
> >  {
> >  	struct pci_p2pmem_client *pos, *tmp;
> >  	struct pci_dev *pdev;
> > +	struct pci_dev *p2pmem = NULL;
> >  
> >  	pdev = find_parent_pci_dev(dev);
> >  	if (!pdev)
> > @@ -392,9 +483,16 @@ void pci_p2pmem_remove_client(struct list_head *head, struct device *dev)
> >  		if (pos->client != pdev)
> >  			continue;
> >  
> > +		if (!p2pmem)
> > +			p2pmem = pci_dev_get(pos->p2pmem);
> > +
> >  		pci_p2pmem_client_free(pos);
> >  	}
> >  
> > +	if (p2pmem && list_empty(head))
> > +		pci_p2pmem_reset_acs(p2pmem);
> > +
> > +	pci_dev_put(p2pmem);
> >  	pci_dev_put(pdev);
> >  }
> >  EXPORT_SYMBOL_GPL(pci_p2pmem_remove_client);
> > @@ -412,6 +510,10 @@ void pci_p2pmem_client_list_free(struct list_head *head)
> >  {
> >  	struct pci_p2pmem_client *pos, *tmp;
> >  
> > +	pos = list_first_entry_or_null(head, struct pci_p2pmem_client, list);
> > +	if (pos && pos->p2pmem)
> > +		pci_p2pmem_reset_acs(pos->p2pmem);
> > +
> >  	list_for_each_entry_safe(pos, tmp, head, list)
> >  		pci_p2pmem_client_free(pos);
> >  }
> > @@ -440,6 +542,40 @@ static bool upstream_bridges_match_list(struct pci_dev *pdev,
> >  	return ret;
> >  }
> >  
> > +static int bind_clients(struct pci_dev *p2pmem, struct list_head *clients)
> > +{
> > +	int ret;
> > +	struct pci_p2pmem_client *pos, *unwind_pos;
> > +
> > +	ret = pci_p2pmem_disable_acs(p2pmem);
> > +	if (ret)
> > +		return ret;
> > +
> > +	list_for_each_entry(pos, clients, list) {
> > +		ret = pci_p2pmem_disable_acs(pos->client);
> > +		if (ret)
> > +			goto unwind;
> > +
> > +		pos->p2pmem = pci_dev_get(p2pmem);
> > +	}
> > +
> > +	return 0;
> > +
> > +unwind:
> > +	list_for_each_entry(unwind_pos, clients, list) {
> > +		if (pos == unwind_pos)
> > +			break;
> > +
> > +		pci_p2pmem_reset_acs(unwind_pos->client);
> > +		pci_dev_put(unwind_pos->p2pmem);
> > +		unwind_pos->p2pmem = NULL;
> > +	}
> > +
> > +	pci_p2pmem_reset_acs(p2pmem);
> > +
> > +	return ret;
> > +}
> > +
> >  /**
> >   * pci_p2pmem_find - find a p2p mem device compatible with the specified device
> >   * @dev: list of device to check (NULL-terminated)
> > @@ -450,11 +586,13 @@ static bool upstream_bridges_match_list(struct pci_dev *pdev,
> >   *
> >   * Returns a pointer to the PCI device with a reference taken (use pci_dev_put
> >   * to return the reference) or NULL if no compatible device is found.
> > + *
> > + * The P2P ACS flags on all applicable PCI devices will be cleared and
> > + * reset when the client is removed from the list.
> >   */
> >  struct pci_dev *pci_p2pmem_find(struct list_head *clients)
> >  {
> >  	struct pci_dev *pdev = NULL;
> > -	struct pci_p2pmem_client *pos;
> >  
> >  	while ((pdev = pci_get_device(PCI_ANY_ID, PCI_ANY_ID, pdev))) {
> >  		if (!pdev->p2p || !pdev->p2p->published)
> > @@ -463,8 +601,8 @@ struct pci_dev *pci_p2pmem_find(struct list_head *clients)
> >  		if (!upstream_bridges_match_list(pdev, clients))
> >  			continue;
> >  
> > -		list_for_each_entry(pos, clients, list)
> > -			pos->p2pmem = pdev;
> > +		if (bind_clients(pdev, clients))
> > +			continue;
> >  
> >  		return pdev;
> >  	}
> > diff --git a/include/linux/pci.h b/include/linux/pci.h
> > index 047aea679e87..cdd4d3c3e562 100644
> > --- a/include/linux/pci.h
> > +++ b/include/linux/pci.h
> > @@ -435,6 +435,8 @@ struct pci_dev {
> >  #endif
> >  #ifdef CONFIG_PCI_P2P
> >  	struct pci_p2p *p2p;
> > +	unsigned int p2p_acs_requests;
> > +	u16 p2p_old_acs_flags;
> >  #endif
> >  	phys_addr_t rom; /* Physical address of ROM if it's not from the BAR */
> >  	size_t romlen; /* Length of ROM if it's not from the BAR */
> > -- 
> > 2.11.0
> >