linux-kernel - Re: [PATCH v4 5/7] PCI/IOV: Check that VF BAR fits within the reservation

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Message-ID: <20241028165604.GA1105091@bhelgaas>
Date: Mon, 28 Oct 2024 11:56:04 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Michał Winiarski <michal.winiarski@...el.com>
Cc: linux-pci@...r.kernel.org, intel-xe@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Christian König <christian.koenig@....com>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.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 v4 5/7] PCI/IOV: Check that VF BAR fits within the
 reservation

On Fri, Oct 25, 2024 at 11:50:36PM +0200, Michał Winiarski wrote:
> VF MMIO resource reservation, either created by system firmware and
> inherited by Linux PCI subsystem or created by the subsystem itself,
> should contain enough space to fit the BAR of all SR-IOV Virtual
> Functions that can potentially be created (total VFs supported by the
> device).

I don't think "VF resource reservation ... should contain enough
space" is really accurate or actionable.  It would be *nice* if the PF
BAR is large enough to accommodate the largest supported VF BARs for
all possible VFs, but if it doesn't, it's not really an error.  It's
just a reflection of the fact that resource space is limited.

> However, that assumption only holds in an environment where VF BAR size
> can't be modified.

There's no reason to assume anything about how many VF BARs fit.  The
existing code should avoid enabling the requested nr_virtfn VFs if the
PF doesn't have enough space -- I think that's what the "if
(res->parent)" is supposed to be checking.

The fact that you need a change here makes me suspect that we're
missing some resource claim (and corresponding res->parent update)
elsewhere when resizing the VF BAR.

> Add an additional check that verifies that VF BAR for all enabled VFs
> fits within the underlying reservation resource.
> 
> Signed-off-by: Michał Winiarski <michal.winiarski@...el.com>
> ---
>  drivers/pci/iov.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> index 79143c1bc7bb4..5de828e5a26ea 100644
> --- a/drivers/pci/iov.c
> +++ b/drivers/pci/iov.c
> @@ -645,10 +645,14 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn)
>  
>  	nres = 0;
>  	for (i = 0; i < PCI_SRIOV_NUM_BARS; i++) {
> +		int vf_bar_sz = pci_iov_resource_size(dev,
> +						      pci_resource_to_iov(i));
>  		bars |= (1 << pci_resource_to_iov(i));
>  		res = &dev->resource[pci_resource_to_iov(i)];
> -		if (res->parent)
> -			nres++;
> +		if (!res->parent || vf_bar_sz * nr_virtfn > resource_size(res))
> +			continue;
> +
> +		nres++;
>  	}
>  	if (nres != iov->nres) {
>  		pci_err(dev, "not enough MMIO resources for SR-IOV\n");
> -- 
> 2.47.0
>