Subject: Re: [PATCH RFC v4 08/21] nvme-pci: Handle movable BARs

From: Bjorn Helgaas <[email-masked]>
Date: Tue, 26 Mar 2019 15:20:55 -0500

[+cc Keith, Jens, Christoph, Sagi, linux-nvme, LKML]

On Mon, Mar 11, 2019 at 04:31:09PM +0300, Sergey Miroshnichenko wrote:
> Hotplugged devices can affect the existing ones by moving their BARs.
> PCI subsystem will inform the NVME driver about this by invoking
> reset_prepare()+reset_done(), then iounmap()+ioremap() must be called.

Do you mean the PCI core will invoke ->rescan_prepare() and
->rescan_done() (as opposed to *reset*)?

> Signed-off-by: Sergey Miroshnichenko <[email-masked]>
> ---
>  drivers/nvme/host/pci.c | 29 +++++++++++++++++++++++++++--
>  1 file changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
> index 92bad1c810ac..ccea3033a67a 100644
> --- a/drivers/nvme/host/pci.c
> +++ b/drivers/nvme/host/pci.c
> @@ -106,6 +106,7 @@ struct nvme_dev {
>  	unsigned int num_vecs;
>  	int q_depth;
>  	u32 db_stride;
> +	resource_size_t current_phys_bar;
>  	void __iomem *bar;
>  	unsigned long bar_mapped_size;
>  	struct work_struct remove_work;
> @@ -1672,13 +1673,16 @@ static int nvme_remap_bar(struct nvme_dev *dev, unsigned long size)
>  {
>  	struct pci_dev *pdev = to_pci_dev(dev->dev);
>  
> -	if (size <= dev->bar_mapped_size)
> +	if (dev->bar &&
> +	    dev->current_phys_bar == pci_resource_start(pdev, 0) &&
> +	    size <= dev->bar_mapped_size)
>  		return 0;
>  	if (size > pci_resource_len(pdev, 0))
>  		return -ENOMEM;
>  	if (dev->bar)
>  		iounmap(dev->bar);
> -	dev->bar = ioremap(pci_resource_start(pdev, 0), size);
> +	dev->current_phys_bar = pci_resource_start(pdev, 0);
> +	dev->bar = ioremap(dev->current_phys_bar, size);

dev->current_phys_bar is different from pci_resource_start() in the
case where the PCI core has moved the nvme BAR, but nvme has not yet
remapped it.

I'm not sure it's worth keeping track of current_phys_bar, as opposed
to always unmapping and remapping.  Is this a performance path?  I
think there are advantages to always exercising the same code path,
regardless of whether the BAR happened to be moved, e.g., if there's a
bug in the "BAR moved" path, it may be a heisenbug because whether we
exercise that path depends on the current configuration.

If you do need to cache current_phys_bar, maybe this, so it's a little
easier to see that you're not changing the ioremap() itself:

  dev->bar = ioremap(pci_resource_start(pdev, 0), size);
  dev->current_phys_bar = pci_resource_start(pdev, 0);

>  	if (!dev->bar) {
>  		dev->bar_mapped_size = 0;
>  		return -ENOMEM;
> @@ -2504,6 +2508,8 @@ static void nvme_reset_work(struct work_struct *work)
>  	if (WARN_ON(dev->ctrl.state != NVME_CTRL_RESETTING))
>  		goto out;
>  
> +	nvme_remap_bar(dev, db_bar_size(dev, 0));

How is this change connected to rescan?  This looks reset-related.

>  	/*
>  	 * If we're called to reset a live controller first shut it down before
>  	 * moving on.
> @@ -2910,6 +2916,23 @@ static void nvme_error_resume(struct pci_dev *pdev)
>  	flush_work(&dev->ctrl.reset_work);
>  }
>  
> +void nvme_rescan_prepare(struct pci_dev *pdev)
> +{
> +	struct nvme_dev *dev = pci_get_drvdata(pdev);
> +
> +	nvme_dev_disable(dev, false);
> +	nvme_dev_unmap(dev);
> +	dev->bar = NULL;
> +}
> +
> +void nvme_rescan_done(struct pci_dev *pdev)
> +{
> +	struct nvme_dev *dev = pci_get_drvdata(pdev);
> +
> +	nvme_dev_map(dev);
> +	nvme_reset_ctrl_sync(&dev->ctrl);
> +}
> +
>  static const struct pci_error_handlers nvme_err_handler = {
>  	.error_detected	= nvme_error_detected,
>  	.slot_reset	= nvme_slot_reset,
> @@ -2974,6 +2997,8 @@ static struct pci_driver nvme_driver = {
>  	},
>  	.sriov_configure = pci_sriov_configure_simple,
>  	.err_handler	= &nvme_err_handler,
> +	.rescan_prepare	= nvme_rescan_prepare,
> +	.rescan_done	= nvme_rescan_done,
>  };
>  
>  static int __init nvme_init(void)
> -- 
> 2.20.1
>