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

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 2 Nov 2022 18:42:21 -0500
Bjorn Helgaas <helgaas@kernel.org> 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.
> 
> 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));

Do you want to test the return value here to avoid another case of not
actually doing what we expect it to do?  WARN_ON perhaps?  Thanks,

Alex

> > +			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.  
>