linux-kernel - Re: [PATCH v2 1/1] PCI: Fix BAR resizing when VF BARs are assigned

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Message-ID: <20250318092352.1bf43af4.alex.williamson@redhat.com>
Date: Tue, 18 Mar 2025 09:23:52 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: Jakub Kicinski <kuba@...nel.org>, Alexander Duyck
 <alexanderduyck@...com>, Michał Winiarski
 <michal.winiarski@...el.com>, Bjorn Helgaas <bhelgaas@...gle.com>,
 Christian König <christian.koenig@....com>,
 linux-pci@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] PCI: Fix BAR resizing when VF BARs are assigned

On Tue, 18 Mar 2025 09:01:57 -0600
Alex Williamson <alex.williamson@...hat.com> wrote:

> On Tue, 18 Mar 2025 13:42:57 +0200 (EET)
> Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com> wrote:
> 
> > + Jakub
> > + Alexander
> > 
> > On Mon, 17 Mar 2025, Alex Williamson wrote:  
> > > On Mon, 17 Mar 2025 19:18:03 +0100
> > > Michał Winiarski <michal.winiarski@...el.com> wrote:    
> > > > On Fri, Mar 14, 2025 at 08:56:49AM -0600, Alex Williamson wrote:    
> > > > > On Fri,  7 Mar 2025 16:03:49 +0200
> > > > > Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com> wrote:
> > > > >       
> > > > > > __resource_resize_store() attempts to release all resources of the
> > > > > > device before attempting the resize. The loop, however, only covers
> > > > > > standard BARs (< PCI_STD_NUM_BARS). If a device has VF BARs that are
> > > > > > assigned, pci_reassign_bridge_resources() finds the bridge window still
> > > > > > has some assigned child resources and returns -NOENT which makes
> > > > > > pci_resize_resource() to detect an error and abort the resize.
> > > > > > 
> > > > > > Change the release loop to cover all resources up to VF BARs which
> > > > > > allows the resize operation to release the bridge windows and attempt
> > > > > > to assigned them again with the different size.
> > > > > > 
> > > > > > As __resource_resize_store() checks first that no driver is bound to
> > > > > > the PCI device before resizing is allowed, SR-IOV cannot be enabled
> > > > > > during resize so it is safe to release also the IOV resources.      
> > > > > 
> > > > > Is this true?  pci-pf-stub doesn't teardown SR-IOV on release, which I
> > > > > understand is done intentionally.  Thanks,      
> > 
> > Thanks for reviewing. I'm sorry I just took Michał's word on this for 
> > granted so I didn't check it myself.
> > 
> > I could amend __resource_resize_store() to return -EBUSY if SR-IOV is 
> > there despite no driver being present  
> 
> I probably never really considered resizing BARs for an SR-IOV capable
> device when adding this support originally, but it seems valid to me
> that if we extend releasing resources to the SR-IOV BARs that we simply
> need to assert that SR-IOV is disabled and fail otherwise.  Thanks,

Also, I've never seen it, but I'm under the impression that it's
possible for pre-boot to hand-off a device with SR-IOV already enabled.
Therefore I think this would be a worthwhile addition, regardless of the
behavior of any given in-kernel driver.  Thanks,

Alex