linux-kernel - Re: [PATCH v3 -tip x86/apic 2/2] x86/MSI: Conserve interrupt resources when using multiple-MSIs

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Date:	Wed, 5 Jun 2013 22:08:07 +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 2/2] x86/MSI: Conserve interrupt
 resources when using multiple-MSIs

On Mon, May 13, 2013 at 11:06:17AM +0200, Alexander Gordeev wrote:
> Current multiple-MSI implementation does not take into account
> actual number of requested MSIs and always rounds that number
> to a closest power-of-two value. Yet, a number of MSIs a PCI
> device could send (and therefore a number of messages a device
> driver could request) may be a lesser power-of-two. As result,
> resources allocated for extra MSIs just wasted.
> 
> This update takes advantage of 'msi_desc::nvec' field introduced
> with generic MSI code to track number of requested and used MSIs.
> As result, resources associated with interrupts are conserved.
> Of those resources most noticeable are x86 interrupt vectors.
> 
> The initial version of this fix also consumed on IRTEs, but Jan
> noticed that a malfunctioning PCI device might send a message
> number it did not claim and thus refer an IRTE it does not own.
> To avoid this security hole the old-way approach preserved and
> as many IRTEs are reserved as the device could possibly send.
> 
> Signed-off-by: Alexander Gordeev <agordeev@...hat.com>

This does not look all that bad.

Sebastian
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