Re: [PATCH v9 08/13] PCI: Save and restore the ACS Control register

[Alex Williamson <alex@shazbot.org>]

On Fri, 18 Sep 2026 20:06:34 +0000
David Matlack <dmatlack@google.com> wrote:

> Save the ACS Control register in pci_save_state() and write it back
> in pci_restore_state(), instead of recomputing the ACS controls from
> scratch with pci_enable_acs().
> 
> This makes ACS symmetric with the rest of a device's saved state. Today
> pci_save_state() ignores ACS entirely and pci_restore_state() re-enables
> the ACS controls from the kernel's current ACS policy. As a result, a
> device can come out of a reset with different ACS controls than it went
> in with, e.g. any controls programmed outside of pci_enable_acs() are
> silently dropped.

Controls, yes, but if we're restoring controls that might have EC set
now, should the Egress Control Vector also be part of the save state?

Currently EC always gets cleared on reset and won't be restored by
pci_enable_acs(), so we can lose both EC and the EC vector.  With this,
I think we restore EC but still lose the EC vector.  Thanks,

Alex

> pci_enable_acs() runs when a driver binds to a device
> (pci_dma_configure()), i.e. after pci_bus_add_device() has already saved
> the device's state. Refresh the saved ACS Control register there as
> well, otherwise a subsequent reset would revert ACS back to the
> configuration left behind by firmware.
> 
> Devices that rely on device-specific quirks to enable an ACS equivalent
> keep that configuration outside of the ACS Control register, so keep
> configuring ACS from scratch for them. Do the same for devices that have
> no saved ACS state at all.
> 
> Reviewed-by: Bjorn Helgaas <bhelgaas@google.com>
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
>  drivers/pci/pci.c    | 66 +++++++++++++++++++++++++++++++++++++++++++-
>  drivers/pci/pci.h    |  5 ++++
>  drivers/pci/quirks.c |  7 +++++
>  3 files changed, 77 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index b2879a6be5f8..dd25c01736b4 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -1021,6 +1021,55 @@ static void pci_std_enable_acs(struct pci_dev *dev, struct pci_acs *caps)
>  		caps->ctrl |= (dev->acs_capabilities & PCI_ACS_TB);
>  }
>  
> +/**
> + * pci_save_acs_state - save the ACS Control register
> + * @dev: the PCI device
> + *
> + * Record the ACS controls currently programmed in hardware so that
> + * pci_restore_acs_state() can reapply them after a reset.
> + */
> +static void pci_save_acs_state(struct pci_dev *dev)
> +{
> +	struct pci_cap_saved_state *save_state;
> +
> +	if (!dev->acs_cap)
> +		return;
> +
> +	save_state = pci_find_saved_ext_cap(dev, PCI_EXT_CAP_ID_ACS);
> +	if (!save_state)
> +		return;
> +
> +	pci_read_config_word(dev, dev->acs_cap + PCI_ACS_CTRL,
> +			     (u16 *)&save_state->cap.data[0]);
> +}
> +
> +/**
> + * pci_restore_acs_state - restore the ACS Control register
> + * @dev: the PCI device
> + */
> +static void pci_restore_acs_state(struct pci_dev *dev)
> +{
> +	struct pci_cap_saved_state *save_state = NULL;
> +
> +	if (dev->acs_cap && !pci_need_dev_specific_enable_acs(dev))
> +		save_state = pci_find_saved_ext_cap(dev, PCI_EXT_CAP_ID_ACS);
> +
> +	/*
> +	 * Devices that rely on device-specific quirks to enable an ACS
> +	 * equivalent keep that configuration outside of the ACS Control
> +	 * register, so there is nothing useful to restore for them. Configure
> +	 * ACS from scratch instead, which also covers devices that have no
> +	 * saved ACS state at all.
> +	 */
> +	if (!save_state) {
> +		pci_enable_acs(dev);
> +		return;
> +	}
> +
> +	pci_write_config_word(dev, dev->acs_cap + PCI_ACS_CTRL,
> +			      *(u16 *)&save_state->cap.data[0]);
> +}
> +
>  /**
>   * pci_enable_acs - enable ACS if hardware support it
>   * @dev: the PCI device
> @@ -1057,6 +1106,15 @@ void pci_enable_acs(struct pci_dev *dev)
>  	__pci_config_acs(dev, &caps, config_acs_param, 0, 0);
>  
>  	pci_write_config_word(dev, pos + PCI_ACS_CTRL, caps.ctrl);
> +
> +	/*
> +	 * pci_enable_acs() runs when a driver binds to the device, i.e. after
> +	 * pci_bus_add_device() has already saved the device's state. Refresh
> +	 * the saved ACS Control register so that a subsequent reset restores
> +	 * the controls programmed here rather than the ones left behind by
> +	 * firmware.
> +	 */
> +	pci_save_acs_state(dev);
>  }
>  
>  /**
> @@ -1800,6 +1858,7 @@ int pci_save_state(struct pci_dev *dev)
>  	pci_save_aer_state(dev);
>  	pci_save_ptm_state(dev);
>  	pci_save_tph_state(dev);
> +	pci_save_acs_state(dev);
>  	return pci_save_vc_state(dev);
>  }
>  EXPORT_SYMBOL(pci_save_state);
> @@ -1877,7 +1936,7 @@ void pci_restore_state(struct pci_dev *dev)
>  	pci_restore_msi_state(dev);
>  
>  	/* Restore ACS and IOV configuration state */
> -	pci_enable_acs(dev);
> +	pci_restore_acs_state(dev);
>  	pci_restore_iov_state(dev);
>  
>  	dev->state_saved = false;
> @@ -3532,6 +3591,11 @@ void pci_allocate_cap_save_buffers(struct pci_dev *dev)
>  	if (error)
>  		pci_err(dev, "unable to allocate suspend buffer for LTR\n");
>  
> +	error = pci_add_ext_cap_save_buffer(dev, PCI_EXT_CAP_ID_ACS,
> +					    sizeof(u16));
> +	if (error)
> +		pci_err(dev, "unable to allocate suspend buffer for ACS\n");
> +
>  	pci_allocate_vc_save_buffers(dev);
>  }
>  
> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> index ba3c3fddddc2..037c1674f164 100644
> --- a/drivers/pci/pci.h
> +++ b/drivers/pci/pci.h
> @@ -1095,6 +1095,7 @@ void pci_acs_init(struct pci_dev *dev);
>  void pci_enable_acs(struct pci_dev *dev);
>  #ifdef CONFIG_PCI_QUIRKS
>  int pci_dev_specific_acs_enabled(struct pci_dev *dev, u16 acs_flags);
> +bool pci_need_dev_specific_enable_acs(struct pci_dev *dev);
>  int pci_dev_specific_enable_acs(struct pci_dev *dev);
>  int pci_dev_specific_disable_acs_redir(struct pci_dev *dev);
>  void pci_disable_broken_acs_cap(struct pci_dev *pdev);
> @@ -1105,6 +1106,10 @@ static inline int pci_dev_specific_acs_enabled(struct pci_dev *dev,
>  {
>  	return -ENOTTY;
>  }
> +static inline bool pci_need_dev_specific_enable_acs(struct pci_dev *dev)
> +{
> +	return false;
> +}
>  static inline int pci_dev_specific_enable_acs(struct pci_dev *dev)
>  {
>  	return -ENOTTY;
> diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
> index 7aee30734303..e500c202d2ec 100644
> --- a/drivers/pci/quirks.c
> +++ b/drivers/pci/quirks.c
> @@ -5476,6 +5476,13 @@ static const struct pci_dev_acs_ops *pci_dev_acs_ops_get(struct pci_dev *dev)
>  	return NULL;
>  }
>  
> +bool pci_need_dev_specific_enable_acs(struct pci_dev *dev)
> +{
> +	const struct pci_dev_acs_ops *p = pci_dev_acs_ops_get(dev);
> +
> +	return p && p->enable_acs;
> +}
> +
>  int pci_dev_specific_enable_acs(struct pci_dev *dev)
>  {
>  	const struct pci_dev_acs_ops *p = pci_dev_acs_ops_get(dev);