Re: [PATCH V2] PCI: vmd: Fix secondary bus reset for Intel bridges

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Alex, Myron]

On Mon, Oct 31, 2022 at 02:45:01PM -0700, francisco.munoz.ruiz@linux.intel.com wrote:
> From: Francisco Munoz <francisco.munoz.ruiz@linux.intel.com>
> 
> The reset was never applied in the current implementation because Intel
> Bridges owned by VMD are parentless. Internally, pci_reset_bus applies
> a reset to the parent of the pci device supplied as argument, but in this
> case it failed because there wasn't a parent.
> 
> In more detail, this change allows the VMD driver to enumerate NVMe devices
> in pass-through configurations when host reboots are performed. Commit id
> “6aab5622296b990024ee67dd7efa7d143e7558d0” attempted to fix this, but
> later we discovered that the code inside pci_reset_bus wasn’t triggering
> secondary bus resets.  Therefore, we updated the parameters passed to
> it, and now NVMe SSDs attached to VMD bridges are properly enumerated in
> VT-d pass-through scenarios.

Did you mean "guest reboots" above?  If the *host* reboots, I assume
everybody (host and guests) starts over, so a reset wouldn't really
apply.

Is the scenario that the VMD device is passed through to a guest, and
the guest OS is running vmd_probe() and vmd_enable_domain()?

I thought VFIO already had something to reset devices between guests.
But maybe this is different because from the point of view of VFIO,
the pass-through happens only once, and during that single session,
the guest OS reboots several times, so you want vmd_probe() to reset
the downstream devices?

Should this have a Fixes: tag for 6aab5622296b?

s/pci/PCI/ above in English text.

Also add "()" after function names.

Use the typical 12-char SHA1 + subject citation, e.g., 6aab5622296b
("PCI: vmd: Clean up domain before enumeration").

> Signed-off-by: Francisco Munoz <francisco.munoz.ruiz@linux.intel.com>
> Reviewed-by: Nirmal Patel <nirmal.patel@linux.intel.com>
> ---
>  drivers/pci/controller/vmd.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/controller/vmd.c b/drivers/pci/controller/vmd.c
> index e06e9f4fc50f..34d6ba675440 100644
> --- a/drivers/pci/controller/vmd.c
> +++ b/drivers/pci/controller/vmd.c
> @@ -859,8 +859,16 @@ static int vmd_enable_domain(struct vmd_dev *vmd, unsigned long features)
>  
>  	pci_scan_child_bus(vmd->bus);
>  	vmd_domain_reset(vmd);
> -	list_for_each_entry(child, &vmd->bus->children, node)
> -		pci_reset_bus(child->self);
> +
> +	list_for_each_entry(child, &vmd->bus->children, node) {
> +		if (!list_empty(&child->devices)) {
> +			pci_reset_bus(list_first_entry(&child->devices,
> +						       struct pci_dev,
> +						       bus_list));
> +			break;
> +		}
> +	}
> +
>  	pci_assign_unassigned_bus_resources(vmd->bus);
>  
>  	/*
> -- 
> 2.25.1
> 
> Hi Bjorn,
> 
> I updated the commit message with more details. Hopefully, this will 
> clarify its purpose.
> 
> Thanks,
> Francisco.