linux-kernel - Re: [PATCH v2 1/1] PCI/IOV: Fix incorrect cfg

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Message-ID: <1560369152.22674.6.camel@amazon.de>
Date:   Wed, 12 Jun 2019 19:52:32 +0000
From:   "Raslan, KarimAllah" <karahmed@...zon.de>
To:     "ashok.raj@...el.com" <ashok.raj@...el.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>
CC:     "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "keith.busch@...el.com" <keith.busch@...el.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "mike.campin@...el.com" <mike.campin@...el.com>
Subject: Re: [PATCH v2 1/1] PCI/IOV: Fix incorrect cfg_size for VF > 0

On Wed, 2019-06-12 at 12:03 -0700, Raj, Ashok 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@...ux.intel.com>
> > wrote:
> > 
> > > 
> > > On 6/12/19 11:19 AM, Alex Williamson wrote:
> > > > 
> > > > On Wed, 12 Jun 2019 10:06:47 -0700
> > > > sathyanarayanan.kuppuswamy@...ux.intel.com wrote:
> > > >  
> > > > > 
> > > > > From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.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@...el.com>
> > > > > Suggested-by: Mike Campin <mike.campin@...el.com>
> > > > > Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.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.

The original patch was to save time when you have hundreds of VFs in the system 
and doing this for each one of them is just a waste of time.

> 
> + Karim
> 
> Cheers,
> Ashok



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