Re: [PATCH v4 09/10] PCI: save/restore CXL config around reset

[Alex Williamson <alex@shazbot.org>]

On Thu, 22 Jan 2026 10:47:45 +0000
Jonathan Cameron <jonathan.cameron@huawei.com> wrote:

> On Thu, 22 Jan 2026 11:01:57 +0100
> Lukas Wunner <lukas@wunner.de> wrote:
> 
> > On Tue, Jan 20, 2026 at 10:26:09PM +0000, smadhavan@nvidia.com wrote:  
> > > +++ b/drivers/pci/pci.c
> > > @@ -4989,6 +4990,11 @@ static int cxl_reset(struct pci_dev *dev, bool probe)
> > >  	if (probe)
> > >  		return 0;
> > > 
> > > +	pci_save_state(dev);
> > > +	rc = cxl_config_save_state(dev, &cxl_state);
> > > +	if (rc)
> > > +		pci_warn(dev, "Failed to save CXL config state: %d\n", rc);
> > > +    
> > 
> > Hm, shouldn't the call to cxl_config_save_state() be moved to
> > pci_save_state() (and likewise, cxl_config_restore_state() moved to
> > pci_restore_state())?
> > 
> > E.g. when a DPC event occurs, I assume CXL registers need to
> > be restored as well on recovery, right?  
> The CXL spec has some comic language around DPC that basically says
> "use with care, DPC trigger will bring down physical link, reset devicestate,
> disrupt CXL.cache and CXL.mem traffic".
> or in shorter words
> 'Good luck'
> 
> If a CXL device undergoes DPC high chance you'll either trigger CXL isolation
> which we aren't handing yet in Linux because we aren't convinced software
> can really recover form it, or stall a CPU and end up rebooting.
> 
> Maybe we'll one day we'll figure this out. Today turn off DPC on CXL ports! :)

Even if we hand-wave that DPC isn't an issue, save/restore of the PCI
state happens at a higher level for every other PCI reset method and
we're creating inconsistency here.

PCI-core includes interfaces for saving PCI state, offloading PCI state
as an opaque blob, reloading, and restoring that state, and performing
resets without saving and restoring state.  This has a couple users,
including vfio.

If we want similar behavior for CXL type2 devices for a future vfio use
case, we shouldn't create unnecessary differentiation here with saving
the CXL state separately and making the reset method behave
differently.  Thanks,

Alex