linux-kernel - Re: [PATCH 13/21] MSI: Do MSI masking in the arch code

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Date:	Wed, 28 Mar 2007 00:39:21 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Michael Ellerman <michael@...erman.id.au>
Cc:	linux-pci@...ey.karlin.mff.cuni.cz,
	Greg Kroah-Hartman <greg@...ah.com>,
	"David S. Miller" <davem@...emloft.net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	<linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...l.org>,
	<daniel.e.wolstenholme@...el.com>
Subject: Re: [PATCH 13/21] MSI: Do MSI masking in the arch code

Michael Ellerman <michael@...erman.id.au> writes:

> It's an arch detail whether MSI irqs need to be masked using the PCI
> MSI registers.

Agreed.  It isn't an arch detail that they need to be unmasked in
the pci configuration space.

I assume this patch is motivated just to make arch support easier
and not for RTAS compatibility reason.

> This changes behaviour in that previously we unconditionally masked
> all MSIs, eventhough we only ever enable one, whereas now we only
> mask the irqs we're using.
>
> To be super paranoid I have the archs mask the irq before they write
> the msi message, just in case the device doesn't respect the MSI enable
> bit or MSI is already enabled or something else crazy.
>
> For MSI-X this might mean we mask the already masked MSI-X on the device,
> but that should be harmless.

I don't think this patch really makes sense.  I think we should mask
all possible vectors for msi and msi-x initially in the generic code
and then unmask them.

If we were trying to support Dave Miller's example of hypervisors
that don't let us touch the msi registers I think there would be a
point.  As it is I think this just makes the code more brittle.

Eric
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