Re: [PATCH v2 1/1] PCI/IOV: Fix incorrect cfg_size for VF > 0

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

On Wed, 12 Jun 2019 12:03:03 -0700
"Raj, Ashok" <ashok.raj@intel.com> wrote:

> On Wed, Jun 12, 2019 at 12:58:17PM -0600, Alex Williamson wrote:
> > On Wed, 12 Jun 2019 11:41:36 -0700
> > sathyanarayanan kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com>
> > wrote:
> >   
> > > On 6/12/19 11:19 AM, Alex Williamson wrote:  
> > > > On Wed, 12 Jun 2019 10:06:47 -0700
> > > > sathyanarayanan.kuppuswamy@linux.intel.com wrote:
> > > >    
> > > >> From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> > > >>
> > > >> Commit 975bb8b4dc93 ("PCI/IOV: Use VF0 cached config space size for
> > > >> other VFs") calculates and caches the cfg_size for VF0 device before
> > > >> initializing the pcie_cap of the device which results in using incorrect
> > > >> cfg_size for all VF devices > 0. So set pcie_cap of the device before
> > > >> calculating the cfg_size of VF0 device.
> > > >>
> > > >> Fixes: 975bb8b4dc93 ("PCI/IOV: Use VF0 cached config space size for
> > > >> other VFs")
> > > >> Cc: Ashok Raj <ashok.raj@intel.com>
> > > >> Suggested-by: Mike Campin <mike.campin@intel.com>
> > > >> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> > > >> ---
> > > >>
> > > >> Changes since v1:
> > > >>   * Fixed a typo in commit message.
> > > >>
> > > >>   drivers/pci/iov.c | 1 +
> > > >>   1 file changed, 1 insertion(+)
> > > >>
> > > >> diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> > > >> index 3aa115ed3a65..2869011c0e35 100644
> > > >> --- a/drivers/pci/iov.c
> > > >> +++ b/drivers/pci/iov.c
> > > >> @@ -160,6 +160,7 @@ int pci_iov_add_virtfn(struct pci_dev *dev, int id)
> > > >>   	virtfn->device = iov->vf_device;
> > > >>   	virtfn->is_virtfn = 1;
> > > >>   	virtfn->physfn = pci_dev_get(dev);
> > > >> +	virtfn->pcie_cap = pci_find_capability(virtfn, PCI_CAP_ID_EXP);
> > > >>   
> > > >>   	if (id == 0)
> > > >>   		pci_read_vf_config_common(virtfn);    
> > > > Why not re-order until after we've setup pcie_cap?
> > > >
> > > > https://lore.kernel.org/linux-pci/20190604143617.0a226555@x1.home/T/#    
> > > 
> > > pci_read_vf_config_common() also caches values for properties like 
> > > class, hdr_type, susbsystem_vendor/device. These values are read/used in 
> > > pci_setup_device(). So if we can use cached values in 
> > > pci_setup_device(), we don't have to read them from registers twice for 
> > > each device.  
> > 
> > Sorry, I missed that dependency, a bit too subtle.  It's still pretty
> > ugly that pci_setup_device()->set_pcie_port_type() is the canonical
> > location for setting pcie_cap and now we need to kludge it earlier.
> > What about the question in the self follow-up to my patch in the link
> > above, can we simply assume 4K config space on a VF?  Thanks,  
> 
> There should be no issue simply reading them once? I don't know
> what that exact optimization saves, unless some broken VFs didn't
> actually expose all the capabilities in config space and this happens
> to workaround the problem.

AIUI the original commit only saves us a few config space reads for VFs
after the first where we test that extended config space is available
and not simply an alias of standard config space.  Thanks,

Alex