linux-kernel - Re: [PATCH] msi: Immediately mask and unmask msi-x irqs.

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Message-ID: <20070403185219.GB15704@linux-os.sc.intel.com>
Date:	Tue, 3 Apr 2007 11:52:19 -0700
From:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-pci@...ey.karlin.mff.cuni.cz, linux-kernel@...r.kernel.org,
	"Kok, Auke-jan H" <auke-jan.h.kok@...el.com>,
	"Williams, Mitch A" <mitch.a.williams@...el.com>
Subject: Re: [PATCH] msi: Immediately mask and unmask msi-x irqs.

On Tue, Apr 03, 2007 at 01:41:49AM -0600, Eric W. Biederman wrote:
> When we mask or unmask a msi-x irqs the writes may be posted because
> we are writing to memory mapped region.  This means the mask and
> unmask don't happen immediately but at some unspecified time in the
> future.  Which is out of sync with how the mask/unmask logic work
> for ioapic irqs.
> 
> The practical result is that we get very subtle and hard to track down
> irq migration bugs.
> 
> This patch performs a read flush after writes to the MSI-X table for mask
> and unmask operations.  Since the SMP affinity is set while the interrupt
> is masked, and since it's unmasked immediately after, no additional flushes
> are required in the various affinity setting routines.

set_msi_irq_affinity() is already doing read_msi_msg(). So the mask operation
before this should atleast get flushed before we modify the irq destination
information.

With this patch however, unmask happens immediately.

> diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> index ad33e01..435c195 100644
> --- a/drivers/pci/msi.c
> +++ b/drivers/pci/msi.c
> @@ -94,6 +94,7 @@ static void msi_set_mask_bit(unsigned int irq, int flag)
>  		int offset = entry->msi_attrib.entry_nr * PCI_MSIX_ENTRY_SIZE +
>  			PCI_MSIX_ENTRY_VECTOR_CTRL_OFFSET;
>  		writel(flag, entry->mask_base + offset);
> +		readl(entry->mask_base + offset);

Don't we need the flush for the PCI_CAP_ID_MSI case aswell.

thanks,
suresh
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