linux-kernel - Re: [PATCH v3 -tip x86/apic 1/2] PCI/MSI: Allocate as many multiple-MSIs as requested

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Message-ID: <20130605205638.GA5816@breakpoint.cc>
Date:	Wed, 5 Jun 2013 22:56:38 +0200
From:	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-pci@...r.kernel.org,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Joerg Roedel <joro@...tes.org>,
	Jan Beulich <JBeulich@...e.com>,
	Ingo Molnar <mingo@...hat.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v3 -tip x86/apic 1/2] PCI/MSI: Allocate as many
 multiple-MSIs as requested

On Mon, May 13, 2013 at 11:05:48AM +0200, Alexander Gordeev wrote:
> Note, although the existing 'msi_desc::multiple' field might seem
> redundant, in fact in does not. In general case the number of MSIs a
> PCI device is initialized with is not necessarily the closest power-
> of-two value of the number of MSIs the device will send. Thus, in
> theory it would not be always possible to derive the former from the
> latter and we need to keep them both, to stress this corner case.
> Besides, since 'msi_desc::multiple' is a bitfield, throwing it out
> would not save us any space.

The last paragraph makes me curious. The only place where 'multiple' is set is
in do_setup_msi_irqs() and this uses the next power of two for it. And since a
device is not enabled twice, it is not overridden.
So it should be possible to compute 'multiple' out of 'nvec' but it saves
cycles not do to so. I agree to keep 'multiple' but your argument does not
seem to make sense.
While nitpicking, 'nvec' might deserve a better comment than 'number of
messages' since it holds the number of allocated interrupts. :)

Sebastian
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