Re: [PATCH] drm/xe: Add force_p2p module/configfs parameter

On Tue, 2026-08-18 at 08:40 -0700, Nathan Ciobanu wrote:
> Add force_p2p module and configfs parameter to give users an option
> to
> allow P2P DMA between GPU cards that are connected off of the root
> bus
> with no switch/bridge in between, as seen in some virtual machine
> topologies (Q35/QEMU, VMware):
> 
>  \-[0000:03]-+-00.0  Intel Corporation [Intel Graphics]
>              \-01.0  Intel Corporation [Intel Graphics]
> 
> or
> 
>  -[0000:00]-+-01.6-[07]----00.0  Intel Graphics
>             \-01.7-[08]----00.0  Intel Graphics
> 
> In these situations the p2pdma distance calculation API would fail
> the
> host-bridge case

Why does it fail. Typically there is a very good reason, like the host
bridge is not whitelisted?


>  and not give a chance to users to take advantage of P2P
> DMA.
> 
> This module/config parameter should be used with caution after
> proving
> that the memory mapped between the two devices is not getting
> corrupted.
> Users need to verify that the platform's physical topology
> allows/facilitates P2P and tests such as the IGT xe_peer2peer pass.
> 
> Assisted-by: Claude:claude-3.5-sonnet
> Signed-off-by: Nathan Ciobanu <[email protected]>
> Cc: Rodrigo Vivi <[email protected]>
> Cc: Thomas Hellström <[email protected]>
> Cc: Matthew Brost <[email protected]>

I don't think we should do this, but rather fix upstream the cases that
should work (probably by whitelisting the host bridges?)

Thanks,
Thomas



> 
> diff --git a/drivers/gpu/drm/xe/xe_configfs.c
> b/drivers/gpu/drm/xe/xe_configfs.c
> index 052cce962161..187352a8b419 100644
> --- a/drivers/gpu/drm/xe/xe_configfs.c
> +++ b/drivers/gpu/drm/xe/xe_configfs.c
> @@ -255,6 +255,39 @@
>   * The created device directories can be removed using ``rmdir``::
>   *
>   *	# rmdir /sys/kernel/config/xe/0000:03:00.0/
> + *
> + * Force P2P DMA
> + * -------------
> + *
> + * This option is useful in virtual machines when the PCIe emulated
> topology
> + * doesn't mimic the physical one and the p2pdma API fails to
> calculate a
> + * distance between nodes due to whitelist checks. In such cases,
> the user can
> + * set the force_p2p module parameter globally or through configfs,
> if needed,
> + * have the option to granularly control that setting on a per-
> device basis.
> + *
> + * By default, force_p2p will have the same value as the
> xe_modparam.force_p2p
> + * which by default is 0.
> + *
> + * The symptoms that point to the P2P issue that this option is
> trying to
> + * address is:
> + * - dmesg errors such as "xe 0000:03:01.0: cannot be used for peer-
> to-peer DMA
> + *   as the client and provider (0000:03:00.0) do not share an
> upstream bridge
> + *   or whitelisted host bridge"
> + * - Running the IGT xe_peer2peer tests result in skips.
> + * - UMD libraries fail with p2p related errors.
> + *
> + * Use these steps if the global xe.force_p2p parameter is
> inconvenient
> + * for some GPU devices in your system. NOTE that this option needs
> to be
> + * set on both endpoints of a pair to take effect.
> + *
> + * 1. Unbind the driver from device:
> + *	# echo 0000:03:00.0 > /sys/bus/pci/drivers/xe/unbind
> + *
> + * 2. Set the force_p2p option on/off:
> + *	# echo [0|1] >
> /sys/kernel/config/xe/0000\:03\:00.0/force_p2p
> + *
> + * 3. Rebind the driver to device:
> + *	# echo 0000:03:00.0 > /sys/bus/pci/drivers/xe/bind
>   */
>  
>  /* Similar to struct xe_bb, but not tied to HW (yet) */
> @@ -275,6 +308,7 @@ struct xe_config_group_device {
>  		bool survivability_mode;
>  		bool enable_psmi;
>  		bool enable_multi_queue;
> +		bool force_p2p;
>  		struct {
>  			unsigned int max_vfs;
>  			bool admin_only_pf;
> @@ -295,6 +329,7 @@ static const struct xe_config_device
> device_defaults = {
>  	.survivability_mode = false,
>  	.enable_psmi = false,
>  	.enable_multi_queue = true,
> +	.force_p2p = XE_DEFAULT_FORCE_P2P,
>  	.sriov = {
>  		.max_vfs = XE_DEFAULT_MAX_VFS,
>  		.admin_only_pf = XE_DEFAULT_ADMIN_ONLY_PF,
> @@ -304,6 +339,7 @@ static const struct xe_config_device
> device_defaults = {
>  static void set_device_defaults(struct xe_config_device *config)
>  {
>  	*config = device_defaults;
> +	config->force_p2p = xe_modparam.force_p2p;
>  #ifdef CONFIG_PCI_IOV
>  	config->sriov.max_vfs = xe_modparam.max_vfs;
>  #endif
> @@ -563,6 +599,37 @@ static ssize_t engines_allowed_store(struct
> config_item *item, const char *page,
>  	return len;
>  }
>  
> +static ssize_t force_p2p_show(struct config_item *item, char *page)
> +{
> +	struct xe_config_group_device *dev =
> to_xe_config_group_device(item);
> +
> +	guard(mutex)(&dev->lock);
> +
> +	return sprintf(page, "%d\n", dev->config.force_p2p);
> +}
> +
> +static ssize_t force_p2p_store(struct config_item *item, const char
> *page, size_t len)
> +{
> +	struct xe_config_group_device *dev =
> to_xe_config_group_device(item);
> +	bool force_p2p;
> +	int ret;
> +
> +	guard(mutex)(&dev->lock);
> +
> +	if (is_bound(dev))
> +		return -EBUSY;
> +
> +	ret = kstrtobool(page, &force_p2p);
> +	if (ret)
> +		return ret;
> +
> +	dev->config.force_p2p = force_p2p;
> +
> +	add_taint(TAINT_USER, LOCKDEP_STILL_OK);
> +
> +	return len;
> +}
> +
>  static ssize_t enable_psmi_show(struct config_item *item, char
> *page)
>  {
>  	struct xe_config_device *dev = to_xe_config_device(item);
> @@ -856,6 +923,7 @@ CONFIGFS_ATTR(, ctx_restore_post_bb);
>  CONFIGFS_ATTR(, enable_multi_queue);
>  CONFIGFS_ATTR(, enable_psmi);
>  CONFIGFS_ATTR(, engines_allowed);
> +CONFIGFS_ATTR(, force_p2p);
>  CONFIGFS_ATTR(, gt_types_allowed);
>  CONFIGFS_ATTR(, survivability_mode);
>  
> @@ -865,6 +933,7 @@ static struct configfs_attribute
> *xe_config_device_attrs[] = {
>  	&attr_enable_multi_queue,
>  	&attr_enable_psmi,
>  	&attr_engines_allowed,
> +	&attr_force_p2p,
>  	&attr_gt_types_allowed,
>  	&attr_survivability_mode,
>  	NULL,
> @@ -1269,6 +1338,25 @@ bool xe_configfs_get_psmi_enabled(struct
> pci_dev *pdev)
>  	return ret;
>  }
>  
> +/** xe_configfs_get_force_p2p - get configfs force_p2p setting
> + * @pdev: pci device
> + *
> + * Return: force_p2p setting in configfs
> + */
> +bool xe_configfs_get_force_p2p(struct pci_dev *pdev)
> +{
> +	struct xe_config_group_device *dev =
> find_xe_config_group_device(pdev);
> +	bool ret;
> +
> +	if (!dev)
> +		return xe_modparam.force_p2p;
> +
> +	ret = dev->config.force_p2p;
> +
> +	config_group_put(&dev->group);
> +	return ret;
> +}
> +
>  /**
>   * xe_configfs_get_enable_multi_queue - get configfs
> enable_multi_queue setting
>   * @pdev: pci device
> diff --git a/drivers/gpu/drm/xe/xe_configfs.h
> b/drivers/gpu/drm/xe/xe_configfs.h
> index 4fbbeafba473..45af69cb74b1 100644
> --- a/drivers/gpu/drm/xe/xe_configfs.h
> +++ b/drivers/gpu/drm/xe/xe_configfs.h
> @@ -30,6 +30,7 @@ u32 xe_configfs_get_ctx_restore_mid_bb(struct
> pci_dev *pdev,
>  u32 xe_configfs_get_ctx_restore_post_bb(struct pci_dev *pdev,
>  					enum xe_engine_class class,
>  					const u32 **cs);
> +bool xe_configfs_get_force_p2p(struct pci_dev *pdev);
>  #ifdef CONFIG_PCI_IOV
>  unsigned int xe_configfs_get_max_vfs(struct pci_dev *pdev);
>  bool xe_configfs_admin_only_pf(struct pci_dev *pdev);
> @@ -50,6 +51,7 @@ static inline u32
> xe_configfs_get_ctx_restore_mid_bb(struct pci_dev *pdev,
>  static inline u32 xe_configfs_get_ctx_restore_post_bb(struct pci_dev
> *pdev,
>  						      enum
> xe_engine_class class,
>  						      const u32
> **cs) { return 0; }
> +static inline bool xe_configfs_get_force_p2p(struct pci_dev *pdev) {
> return xe_modparam.force_p2p; }
>  #ifdef CONFIG_PCI_IOV
>  static inline unsigned int xe_configfs_get_max_vfs(struct pci_dev
> *pdev)
>  {
> diff --git a/drivers/gpu/drm/xe/xe_defaults.h
> b/drivers/gpu/drm/xe/xe_defaults.h
> index 0884224ef7c7..3881e832566a 100644
> --- a/drivers/gpu/drm/xe/xe_defaults.h
> +++ b/drivers/gpu/drm/xe/xe_defaults.h
> @@ -13,6 +13,7 @@
>  #define XE_DEFAULT_GUC_LOG_LEVEL		1
>  #endif
>  
> +#define XE_DEFAULT_FORCE_P2P			0
>  #define
> XE_DEFAULT_PROBE_DISPLAY		IS_ENABLED(CONFIG_DRM_XE_DISPLAY)
>  #define XE_DEFAULT_VRAM_BAR_SIZE		0
>  #define
> XE_DEFAULT_FORCE_PROBE			CONFIG_DRM_XE_FORCE_PROBE
> diff --git a/drivers/gpu/drm/xe/xe_dma_buf.c
> b/drivers/gpu/drm/xe/xe_dma_buf.c
> index bf0728838ead..bd8f8ad70bcd 100644
> --- a/drivers/gpu/drm/xe/xe_dma_buf.c
> +++ b/drivers/gpu/drm/xe/xe_dma_buf.c
> @@ -15,6 +15,7 @@
>  
>  #include "tests/xe_test.h"
>  #include "xe_bo.h"
> +#include "xe_configfs.h"
>  #include "xe_device.h"
>  #include "xe_pm.h"
>  #include "xe_ttm_vram_mgr.h"
> @@ -26,9 +27,14 @@ static int xe_dma_buf_attach(struct dma_buf
> *dmabuf,
>  			     struct dma_buf_attachment *attach)
>  {
>  	struct drm_gem_object *obj = attach->dmabuf->priv;
> +	struct pci_dev *a_pdev = to_pci_dev(obj->dev->dev);
> +	struct pci_dev *b_pdev = dev_is_pci(attach->dev) ?
> +				 to_pci_dev(attach->dev) : NULL;
>  
>  	if (attach->peer2peer &&
> -	    pci_p2pdma_distance(to_pci_dev(obj->dev->dev), attach-
> >dev, false) < 0)
> +	    !(b_pdev && xe_configfs_get_force_p2p(a_pdev) &&
> +	      xe_configfs_get_force_p2p(b_pdev)) &&
> +	    pci_p2pdma_distance(a_pdev, attach->dev, false) < 0)
>  		attach->peer2peer = false;
>  
>  	if (!attach->peer2peer &&
> !xe_bo_can_migrate(gem_to_xe_bo(obj), XE_PL_TT))
> diff --git a/drivers/gpu/drm/xe/xe_module.c
> b/drivers/gpu/drm/xe/xe_module.c
> index 4bc28dfc1992..99cb2a455c6e 100644
> --- a/drivers/gpu/drm/xe/xe_module.c
> +++ b/drivers/gpu/drm/xe/xe_module.c
> @@ -22,6 +22,7 @@
>  #include "xe_sched_job.h"
>  
>  struct xe_modparam xe_modparam = {
> +	.force_p2p =		XE_DEFAULT_FORCE_P2P,
>  	.probe_display =	XE_DEFAULT_PROBE_DISPLAY,
>  	.guc_log_level =	XE_DEFAULT_GUC_LOG_LEVEL,
>  	.force_probe =		XE_DEFAULT_FORCE_PROBE,
> @@ -38,6 +39,10 @@ module_param_named(svm_notifier_size,
> xe_modparam.svm_notifier_size, uint, 0600)
>  MODULE_PARM_DESC(svm_notifier_size, "Set the svm notifier size in
> MiB, must be power of 2 "
>  		 "[default="
> __stringify(XE_DEFAULT_SVM_NOTIFIER_SIZE) "]");
>  
> +module_param_named_unsafe(force_p2p, xe_modparam.force_p2p, bool,
> 0400);
> +MODULE_PARM_DESC(force_p2p, "Force P2P DMA between GPUs (useful in
> virtual machines), bypassing p2pdma checks. "
> +		 "[default=" __stringify(XE_DEFAULT_FORCE_P2P) "]");
> +
>  #if IS_ENABLED(CONFIG_DRM_XE_DISPLAY)
>  module_param_named(probe_display, xe_modparam.probe_display, bool,
> 0444);
>  MODULE_PARM_DESC(probe_display, "Probe display HW, otherwise it's
> left untouched "
> diff --git a/drivers/gpu/drm/xe/xe_module.h
> b/drivers/gpu/drm/xe/xe_module.h
> index 6272d9e41207..bb154c7b095a 100644
> --- a/drivers/gpu/drm/xe/xe_module.h
> +++ b/drivers/gpu/drm/xe/xe_module.h
> @@ -12,6 +12,7 @@ struct work_struct;
>  
>  /* Module modprobe variables */
>  struct xe_modparam {
> +	bool force_p2p;
>  	bool probe_display;
>  	int force_vram_bar_size;
>  	int guc_log_level;
> diff --git a/drivers/gpu/drm/xe/xe_svm.c
> b/drivers/gpu/drm/xe/xe_svm.c
> index 627a741293d5..3cd53bb166af 100644
> --- a/drivers/gpu/drm/xe/xe_svm.c
> +++ b/drivers/gpu/drm/xe/xe_svm.c
> @@ -12,6 +12,7 @@
>  
>  #include "xe_bo.h"
>  #include "xe_exec_queue_types.h"
> +#include "xe_configfs.h"
>  #include "xe_gt_stats.h"
>  #include "xe_migrate.h"
>  #include "xe_module.h"
> @@ -892,6 +893,10 @@ static bool xe_has_interconnect(struct
> drm_pagemap_peer *peer1,
>  	if (dev1 == dev2)
>  		return true;
>  
> +	if (xe_configfs_get_force_p2p(to_pci_dev(dev1)) &&
> +	    xe_configfs_get_force_p2p(to_pci_dev(dev2)))
> +		return true;
> +
>  	return pci_p2pdma_distance(to_pci_dev(dev1), dev2, true) >=
> 0;
>  }
>