Re: [PATCH v2 04/10] PCI/P2PDMA: Clear ACS P2P flags for all devices behind switches

Bjorn Helgaas
2018-03-01T18:02:57Z

On Wed, Feb 28, 2018 at 04:40:00PM -0700, Logan Gunthorpe wrote:
> For peer-to-peer transactions to work the downstream ports in each
> switch must not have the ACS flags set. At this time there is no way
> to dynamically change the flags and update the corresponding IOMMU
> groups so this is done at enumeration time before the the groups are
> assigned.

s/the the/the/

> This effectively means that if CONFIG_PCI_P2PDMA is selected then
> all devices behind any switch will be in the same IOMMU group.
>
> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> ---
> drivers/pci/Kconfig | 4 ++++
> drivers/pci/p2pdma.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
> drivers/pci/pci.c | 4 ++++
> include/linux/pci-p2pdma.h | 5 +++++
> 4 files changed, 57 insertions(+)
>
> diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
> index 840831418cbd..a430672f0ad4 100644
> --- a/drivers/pci/Kconfig
> +++ b/drivers/pci/Kconfig
> @@ -138,6 +138,10 @@ config PCI_P2PDMA
> it's hard to tell which support it with good performance, so
> at this time you will need a PCIe switch.
>
> + Enabling this option will also disable ACS on all ports behind
> + any PCIe switch. This effictively puts all devices behind any
> + switch into the same IOMMU group.

s/effictively/effectively/

Does this really mean "all devices behind the same Root Port"?

What does this mean in terms of device security? I assume it means,
at least, that individual devices can't be assigned to separate VMs.

I don't mind admitting that this patch makes me pretty nervous, and I
don't have a clear idea of what the implications of this are, or how
to communicate those to end users. "The same IOMMU group" is a pretty
abstract idea.

> If unsure, say N.
>
> config PCI_LABEL
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 4e1c81f64b29..61af07acd21a 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -255,6 +255,50 @@ static struct pci_dev *get_upstream_bridge_port(struct pci_dev *pdev)
> return up2;
> }
>
> +/*
> + * pci_p2pdma_disable_acs - disable ACS flags for ports in PCI
> + * bridges/switches
> + * @pdev: device to disable ACS flags for
> + *
> + * The ACS flags for P2P Request Redirect and P2P Completion Redirect need
> + * to be disabled on any downstream port in any switch in order for
> + * the TLPs to not be forwarded up to the RC which is not what we want
> + * for P2P.
> + *
> + * This function is called when the devices are first enumerated and
> + * will result in all devices behind any switch to be in the same IOMMU
> + * group. At this time there is no way to "hotplug" IOMMU groups so we rely
> + * on this largish hammer. If you need the devices to be in separate groups
> + * don't enable CONFIG_PCI_P2PDMA.
> + *
> + * Returns 1 if the ACS bits for this device were cleared, otherwise 0.
> + */
> +int pci_p2pdma_disable_acs(struct pci_dev *pdev)
> +{
> + struct pci_dev *up;
> + int pos;
> + u16 ctrl;
> +
> + up = get_upstream_bridge_port(pdev);
> + if (!up)
> + return 0;
> + pci_dev_put(up);
> +
> + pos = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_ACS);
> + if (!pos)
> + return 0;
> +
> + dev_info(&pdev->dev, "disabling ACS flags for peer-to-peer DMA\n");
> +
> + pci_read_config_word(pdev, pos + PCI_ACS_CTRL, &ctrl);
> +
> + ctrl &= ~(PCI_ACS_RR | PCI_ACS_CR);
> +
> + pci_write_config_word(pdev, pos + PCI_ACS_CTRL, ctrl);
> +
> + return 1;
> +}
> +
> static bool __upstream_bridges_match(struct pci_dev *upstream,
> struct pci_dev *client)
> {
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index f6a4dd10d9b0..95ad3cf288c8 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -16,6 +16,7 @@
> #include <linux/of.h>
> #include <linux/of_pci.h>
> #include <linux/pci.h>
> +#include <linux/pci-p2pdma.h>
> #include <linux/pm.h>
> #include <linux/slab.h>
> #include <linux/module.h>
> @@ -2826,6 +2827,9 @@ static void pci_std_enable_acs(struct pci_dev *dev)
> */
> void pci_enable_acs(struct pci_dev *dev)
> {
> + if (pci_p2pdma_disable_acs(dev))
> + return;

This doesn't read naturally to me. I do see that when
CONFIG_PCI_P2PDMA is not set, pci_p2pdma_disable_acs() does nothing
and returns 0, so we'll go ahead and try to enable ACS as before.

But I think it would be clearer to have an #ifdef CONFIG_PCI_P2PDMA
right here so it's more obvious that we only disable ACS when it's
selected.

> if (!pci_acs_enable)
> return;
>
> diff --git a/include/linux/pci-p2pdma.h b/include/linux/pci-p2pdma.h
> index 126eca697ab3..f537f521f60c 100644
> --- a/include/linux/pci-p2pdma.h
> +++ b/include/linux/pci-p2pdma.h
> @@ -22,6 +22,7 @@ struct block_device;
> struct scatterlist;
>
> #ifdef CONFIG_PCI_P2PDMA
> +int pci_p2pdma_disable_acs(struct pci_dev *pdev);
> int pci_p2pdma_add_resource(struct pci_dev *pdev, int bar, size_t size,
> u64 offset);
> int pci_p2pdma_add_client(struct list_head *head, struct device *dev);
> @@ -41,6 +42,10 @@ int pci_p2pdma_map_sg(struct device *dev, struct scatterlist *sg, int nents,
> void pci_p2pdma_unmap_sg(struct device *dev, struct scatterlist *sg, int nents,
> enum dma_data_direction dir);
> #else /* CONFIG_PCI_P2PDMA */
> +static inline int pci_p2pdma_disable_acs(struct pci_dev *pdev)
> +{
> + return 0;
> +}
> static inline int pci_p2pdma_add_resource(struct pci_dev *pdev, int bar,
> size_t size, u64 offset)
> {
> --
> 2.11.0
>