linux-kernel - Re: [PATCH v2] SR-IOV: correct broken resource alignment calculations

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Date:	Sun, 30 Aug 2009 08:39:01 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Chris Wright <chrisw@...s-sol.org>
Cc:	Matthew Wilcox <matthew@....cx>,
	Chris Wright <chrisw@...s-sol.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 v2] SR-IOV: correct broken resource alignment
 calculations

On Fri, 28 Aug 2009 13:00:06 -0700
Chris Wright <chrisw@...s-sol.org> wrote:

> * Matthew Wilcox (matthew@....cx) wrote:
> > On Fri, Aug 28, 2009 at 12:17:14PM -0700, Chris Wright wrote:
> > > 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.
> 
> 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 thin PCI specific layer over the generic
> resource_alignment() function which is aware of the special nature of
> VF BARs and does sorting and allocation based on the smaller alignment
> requirement.
> 
> I recognize that while resource_alignment is generic, it's basically a
> PCI helper.  An alternative to this patch is to add PCI VF BAR
> specific information to struct resource.  I opted for the extra layer
> rather than adding such PCI specific information to struct resource.
> This does have the slight downside that we don't cache the BAR size
> and re-read for each alignment query (happens a small handful of
> times during boot for each VF BAR).
> 
> Signed-off-by: Chris Wright <chrisw@...s-sol.org>
> Cc: Jesse Barnes <jbarnes@...tuousgeek.org>
> Cc: Ivan Kokshaysky <ink@...assic.park.msu.ru>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Matthew Wilcox <matthew@....cx>
> Cc: Yu Zhao <yu.zhao@...el.com>
> Cc: stable@...nel.org

Yeah, I like this one better.  I've applied it to my for-linus branch;
would be nice to have a Tested-by for it before I send it to Linus...

Thanks,
-- 
Jesse Barnes, Intel Open Source Technology Center
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