linux-kernel - Re: [PATCH v6 4/6] PCI/IOV: Check that VF BAR fits within the reservation

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Message-ID: <pfwjnmsnit7s4vlz2n4zv4hrra262o5cap6vabalqqg6eud7t3@lzwqijbijs4p>
Date: Wed, 2 Apr 2025 12:33:40 +0200
From: Michał Winiarski <michal.winiarski@...el.com>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
CC: <linux-pci@...r.kernel.org>, <intel-xe@...ts.freedesktop.org>,
	<dri-devel@...ts.freedesktop.org>, LKML <linux-kernel@...r.kernel.org>,
	"Bjorn Helgaas" <bhelgaas@...gle.com>, Christian König
	<christian.koenig@....com>, Krzysztof Wilczyński
	<kw@...ux.com>, Rodrigo Vivi <rodrigo.vivi@...el.com>, Michal Wajdeczko
	<michal.wajdeczko@...el.com>, Lucas De Marchi <lucas.demarchi@...el.com>,
	Thomas Hellström <thomas.hellstrom@...ux.intel.com>,
	"Maarten Lankhorst" <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
	<mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie
	<airlied@...il.com>, Simona Vetter <simona@...ll.ch>, Matt Roper
	<matthew.d.roper@...el.com>
Subject: Re: [PATCH v6 4/6] PCI/IOV: Check that VF BAR fits within the
 reservation

On Fri, Mar 28, 2025 at 06:39:29PM +0200, Ilpo Järvinen wrote:
> On Wed, 26 Mar 2025, Ilpo Järvinen wrote:
> 
> > On Thu, 20 Mar 2025, Michał Winiarski wrote:
> > 
> > > When the resource representing VF MMIO BAR reservation is created, its
> > > size is always large enough to accommodate the BAR of all SR-IOV Virtual
> > > Functions that can potentially be created (total VFs). If for whatever
> > > reason it's not possible to accommodate all VFs - the resource is not
> > > assigned and no VFs can be created.
> > > 
> > > The following patch will allow VF BAR size to be modified by drivers at
> > 
> > "The following patch" sounds to be like you're referring to patch that 
> > follows this description, ie., the patch below. "An upcoming change" is 
> > alternative that doesn't suffer from the same problem.
> > 
> > > a later point in time, which means that the check for resource
> > > assignment is no longer sufficient.
> > > 
> > > Add an additional check that verifies that VF BAR for all enabled VFs
> > > fits within the underlying reservation resource.
> > 
> > So this does not solve the case where the initial size was too large to 
> > fix and such VF BARs remain unassigned, right?
> > 
> > > Signed-off-by: Michał Winiarski <michal.winiarski@...el.com>
> > > ---
> > >  drivers/pci/iov.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > > 
> > > diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> > > index cbf335725d4fb..861273ad9a580 100644
> > > --- a/drivers/pci/iov.c
> > > +++ b/drivers/pci/iov.c
> > > @@ -646,8 +646,13 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn)
> > >  
> > >  	nres = 0;
> > >  	for (i = 0; i < PCI_SRIOV_NUM_BARS; i++) {
> > > +		resource_size_t vf_bar_sz =
> > > +			pci_iov_resource_size(dev,
> > > +					      pci_resource_num_from_vf_bar(i));
> > 
> > Please add int idx = pci_resource_num_from_vf_bar(i);
> > 
> > >  		bars |= (1 << pci_resource_num_from_vf_bar(i));
> > >  		res = &dev->resource[pci_resource_num_from_vf_bar(i)];
> > > +		if (vf_bar_sz * nr_virtfn > resource_size(res))
> > > +			continue;
> > 
> > Not directly related to this patch, I suspect this could actually try to 
> > assign an unassigned resource by doing something like this (perhaps in own 
> > patch, it doesn't even need to be part of this series but can be sent 
> > later if you find the suggestion useful):
> > 
> > 		/* Retry assignment if the initial size didn't fit */
> > 		if (!res->parent && pci_assign_resource(res, idx))
> > 			continue;
> > 
> > Although I suspect reset_resource() might have been called for the 
> > resource and IIRC it breaks the resource somehow but it could have been 
> > that IOV resources can be resummoned from that state though thanks to 
> > their size not being stored into the resource itself but comes from iov 
> > structures.
> 
> I realized reset_resource() will zero the flags so it won't work without 
> getting rid of reset_resource() calls first which I've not yet completed. 
> 
> And once I get the rebar sizes included into bridge window sizing 
> algorithm, the default size could possibly be shrunk by the resource
> fitting/assignment code so the resource assignment should no longer fail 
> just because the initial size was too large. So it shouldn't be necessary 
> after that.

Yeah - and even if something fails in the resource constrained
environment, I think the flow used to reassign the PF resource (and
bring back the ability to create VFs) should involve remove -> rescan
(not just VF enabling).

Thanks,
-Michał

> 
> > >  		if (res->parent)
> > >  			nres++;
> > >  	}
> > > 
> > 
> > 
> 
> -- 
>  i.