linux-kernel - Re: [PATCH] Add support for multiple MSI on x86

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Date:	Tue, 8 Mar 2011 22:05:11 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Micha Nelissen <micha@...i.hopto.org>
cc:	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	"Venkatesh Pallipadi (Venki)" <venki@...gle.com>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	Matthew Wilcox <matthew@....cx>
Subject: Re: [PATCH] Add support for multiple MSI on x86

On Fri, 4 Mar 2011, Micha Nelissen wrote:
> Jesse Barnes wrote:
> > On Sun, 13 Feb 2011 21:25:21 +0100
> > Micha Nelissen <micha@...i.hopto.org> wrote:
> > 
> > > Patch is based on earlier patch from Matthew Wilcox.
> > 
> > I like this before when Matthew was working on it too
> 
> If I understood Thomas' concerns well, it sounds like this feature is
> inherently incompatible with big SMP systems. Therefore it has no chance of
> ever being accepted into mainline in whatever form?

First of all MSI is replaced by MSI-X which does not have the
shortcomings of MSI vs. the vector allocation.

Though if we can come up with a scheme which does not create nested
loops over possibly hundreds of CPUs with interrupts disabled and
global locks held, I have no objections.

One possible solution would be to reserve a block of vectors on all
cpus at boot time via a command line option for MSI block
allocations. That would simply use a bitmap protected by a mutex to
lookup a free vector space. That would avoid the whole loop issue and
work for most of the systems which need to deal with such MSI multi
vector devices. Warning: Just an idea :)

Thanks,

	tglx
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