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

["Munoz Ruiz, Francisco" <francisco.munoz.ruiz@linux.intel.com>]

Thanks for including Alex and Myron.

On 11/2/2022 4:42 PM, Bjorn Helgaas wrote:
> [+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.
> 
Correct, I meant guest reboots.
> 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?
Right, this change just makes sure pci_bus_reset(), introduced in 
6aab5622296b ("PCI: vmd: Clean up domain before enumeration"), is sending
pci_bridge_secondary_bus_reset().

I'll issue a v3 with the suggestions below.
> 
> 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.
Thanks!