linux-kernel - Re: Support for multiple MSI

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Message-ID: <m17i35z6fc.fsf@fess.ebiederm.org>
Date:	Wed, 04 Mar 2009 06:52:39 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Matthew Wilcox <matthew@....cx>
Cc:	linux-pci@...r.kernel.org, jbarnes@...tuousgeek.org,
	linux-kernel@...r.kernel.org
Subject: Re: Support for multiple MSI

Matthew Wilcox <matthew@....cx> writes:

> Currently, Linux supports multiple MSI-X interrupts per device, but only
> a single MSI interrupt.  This patch series adds support to the generic
> PCI code for supporting multiple MSI interrupts.  Architectures will
> need to add support for multiple MSIs, and I have a patch to do that for
> x86 (which needs some more work).  Getting this patch series in first
> is important so we can start supporting this interface in drivers and
> architectures independently.

Do we have any benchmarks anywhere that show that multiple msi support
gains us something?

The requirement to allocate a contiguous block of vector numbers worries
me for the x86 implementation.  I don't like the idea of having to deal
with allocations that can fail because of fragmentation.

The fact that we also can not honor the irq affinity properly for multiple
msi also disturbs me.

At a quick skim your patchset is only the generic code without a single
architecture specific implementation so it appears you have not done the
hard work on figuring out how to deal with multiple msi in the real
world.

Given that msi-x does not have any of these issues without data to say
that there is a true gain in supporting multi-msi I don't see the point
of supporting it.

Eric
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