linux-kernel - Re: [PATCH] SR-IOV: correct broken resource alignment calculations

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Message-ID: <20090828195916.GK10360@sequoia.sous-sol.org>
Date:	Fri, 28 Aug 2009 12:59:16 -0700
From:	Chris Wright <chrisw@...s-sol.org>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Chris Wright <chrisw@...s-sol.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Yu Zhao <yu.zhao@...el.com>, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, stable@...nel.org
Subject: Re: [PATCH] SR-IOV: correct broken resource alignment calculations

* Matthew Wilcox (matthew@....cx) wrote:
> On Fri, Aug 28, 2009 at 12:17:14PM -0700, Chris Wright wrote:
> > An SR-IOV capable device includes an SR-IOV PCIe capability which
> > describes the Virtual Function (VF) BAR requirements.  A typical SR-IOV
> > device can support multiple VFs whose BARs must be in a contiguous region,
> > effectively an array of VF BARs.  The BAR reports the size requirement
> > for a single VF.  We calculate the full range needed by simply multiplying
> > the VF BAR size with the number of possible VFs and create a resource
> > spanning the full range.
> > 
> > This all seems sane enough except it artificially inflates the alignment
> > requirement for the VF BAR.  The VF BAR need only be aligned to the size
> > of a single BAR not the contiguous range of VF BARs.  This can cause us
> > to fail to allocate resources for the BAR despite the fact that we
> > actually have enough space.
> > 
> > This patch adds a support for a new resource alignment type,
> > IORESOURCE_VSIZEALIGN, and allows struct resource to keep track of the
> > size requirements of a VF BAR which are smaller than the full resource
> > size.  This could also be done all within the PCI layer w/out bloating
> > struct resource or using the last available bit for alignment types.
> 
> Yes, I think that would be preferable.  We have a *LOT* of resources in
> the kernel, and the embedded folks would not find it funny if they all
> grew in size suddenly.

OK, I'll send that momentarily.  It has one downside which is we re-read
the BAR size multiple times.
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