Re: [PATCH v4 04/14] PCI/P2PDMA: Clear ACS P2P flags for all devices behind switches

Bjorn Helgaas
2018-05-07T23:13:06Z

[+to Alex]

Alex,

Are you happy with this strategy of turning off ACS based on
CONFIG_PCI_P2PDMA? We only check this at enumeration-time and
I don't know if there are other places we would care?

On Mon, Apr 23, 2018 at 05:30:36PM -0600, 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 groups are
> assigned.
>
> This effectively means that if CONFIG_PCI_P2PDMA is selected then
> all devices behind any PCIe switch heirarchy will be in the same IOMMU
> group. Which implies that individual devices behind any switch
> heirarchy will not be able to be assigned to separate VMs because
> there is no isolation between them. Additionally, any malicious PCIe
> devices will be able to DMA to memory exposed by other EPs in the same
> domain as TLPs will not be checked by the IOMMU.
>
> Given that the intended use case of P2P Memory is for users with
> custom hardware designed for purpose, we do not expect distributors
> to ever need to enable this option. Users that want to use P2P
> must have compiled a custom kernel with this configuration option
> and understand the implications regarding ACS. They will either
> not require ACS or will have design the system in such a way that
> devices that require isolation will be separate from those using P2P
> transactions.
>
> Signed-off-by: Logan Gunthorpe <logang-OTvnGxWRz7hWk0Htik3J/w@public.gmane.org>
> ---
> drivers/pci/Kconfig | 9 +++++++++
> drivers/pci/p2pdma.c | 45 ++++++++++++++++++++++++++++++---------------
> drivers/pci/pci.c | 6 ++++++
> include/linux/pci-p2pdma.h | 5 +++++
> 4 files changed, 50 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
> index b2396c22b53e..b6db41d4b708 100644
> --- a/drivers/pci/Kconfig
> +++ b/drivers/pci/Kconfig
> @@ -139,6 +139,15 @@ config PCI_P2PDMA
> transations must be between devices behind the same root port.
> (Typically behind a network of PCIe switches).
>
> + Enabling this option will also disable ACS on all ports behind
> + any PCIe switch. This effectively puts all devices behind any
> + switch heirarchy into the same IOMMU group. Which implies that

s/heirarchy/hierarchy/ (also above in changelog)

> + individual devices behind any switch will not be able to be
> + assigned to separate VMs because there is no isolation between
> + them. Additionally, any malicious PCIe devices will be able to
> + DMA to memory exposed by other EPs in the same domain as TLPs
> + will not be checked by the IOMMU.
> +
> If unsure, say N.
>
> config PCI_LABEL
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index ed9dce8552a2..e9f43b43acac 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -240,27 +240,42 @@ static struct pci_dev *find_parent_pci_dev(struct device *dev)
> }
>
> /*
> - * 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
> - * for the next level in the hierarchy. Because of this, devices connected
> - * to the root port will be rejected.
> + * pci_p2pdma_disable_acs - disable ACS flags for all PCI bridges
> + * @pdev: device to disable ACS flags for
> + *
> + * The ACS flags for P2P Request Redirect and P2P Completion Redirect need
> + * to be disabled on any PCI bridge in order for the TLPS to not be forwarded
> + * up to the RC which is not what we want for P2P.

s/PCI bridge/PCIe switch/ (ACS doesn't apply to conventional PCI)

> + *
> + * This function is called when the devices are first enumerated and
> + * will result in all devices behind any bridge 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 was cleared, otherwise 0.
> */
> -static struct pci_dev *get_upstream_bridge_port(struct pci_dev *pdev)
> +int pci_p2pdma_disable_acs(struct pci_dev *pdev)
> {
> - struct pci_dev *up1, *up2;
> + int pos;
> + u16 ctrl;
>
> - if (!pdev)
> - return NULL;
> + if (!pci_is_bridge(pdev))
> + return 0;
>
> - up1 = pci_dev_get(pci_upstream_bridge(pdev));
> - if (!up1)
> - return NULL;
> + pos = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_ACS);
> + if (!pos)
> + return 0;
> +
> + pci_info(pdev, "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);
>
> - up2 = pci_dev_get(pci_upstream_bridge(up1));
> - pci_dev_put(up1);
> + pci_write_config_word(pdev, pos + PCI_ACS_CTRL, ctrl);
>
> - return up2;
> + return 1;
> }
>
> /*
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index e597655a5643..7e2f5724ba22 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>
> @@ -2835,6 +2836,11 @@ static void pci_std_enable_acs(struct pci_dev *dev)
> */
> void pci_enable_acs(struct pci_dev *dev)
> {
> +#ifdef CONFIG_PCI_P2PDMA
> + if (pci_p2pdma_disable_acs(dev))
> + return;
> +#endif
> +
> if (!pci_acs_enable)
> return;
>
> diff --git a/include/linux/pci-p2pdma.h b/include/linux/pci-p2pdma.h
> index 0cde88341eeb..fcb3437a2f3c 100644
> --- a/include/linux/pci-p2pdma.h
> +++ b/include/linux/pci-p2pdma.h
> @@ -18,6 +18,7 @@ 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);
> @@ -40,6 +41,10 @@ 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
>