linux-kernel - Re: [PATCH 2.6.21-rc5] MSI: read-flush MSI-X table

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Date:	Tue, 27 Mar 2007 09:25:57 -0600
From:	Grant Grundler <grundler@...isc-linux.org>
To:	Mitch Williams <mitch.a.williams@...el.com>
Cc:	linux-pci@...ey.karlin.mff.cuni.cz, gregkh@...e.de,
	ebiederm@...ssion.com, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, auke-jan.h.kok@...el.com
Subject: Re: [PATCH 2.6.21-rc5] MSI: read-flush MSI-X table

On Mon, Mar 26, 2007 at 04:18:22PM -0700, Mitch Williams wrote:
> This patch fixes a kernel bug which is triggered when using the
> irqbalance daemon with MSI-X hardware.
> 
> Because both MSI-X interrupt messages and MSI-X table writes are posted,
> it's possible for them to cross while in-flight.  This results in
> interrupts being received long after the kernel thinks they're disabled,
> and in interrupts being sent to stale vectors after rebalancing.
>
> This patch performs a read flush after writes to the MSI-X table for
> enable/disable and rebalancing operations.

Why wouldn't MSI have the same problems as MSI-X?

...
> diff -urpN -X dontdiff linux-2.6.21-rc4-clean/drivers/pci/msi.c linux-2.6.21-rc4/drivers/pci/msi.c
> --- linux-2.6.21-rc4-clean/drivers/pci/msi.c	2007-03-19 16:16:32.000000000 -0700
> +++ linux-2.6.21-rc4/drivers/pci/msi.c	2007-03-21 12:44:51.000000000 -0700
> @@ -68,6 +68,29 @@ static void msix_set_enable(struct pci_d
>  	}
>  }
>  
> +void msix_flush_writes(unsigned int irq)
> +{
> +	struct msi_desc *entry;
> +
> +	entry = get_irq_msi(irq);
> +	BUG_ON(!entry || !entry->dev);
> +	switch (entry->msi_attrib.type) {
> +	case PCI_CAP_ID_MSI:
> +		/* nothing to do */
> +		break;
> +	case PCI_CAP_ID_MSIX:
> +	{
> +		int offset = entry->msi_attrib.entry_nr * PCI_MSIX_ENTRY_SIZE +
> +			PCI_MSIX_ENTRY_VECTOR_CTRL_OFFSET;
> +		readl(entry->mask_base + offset);
> +		break;
> +	}
> +	default:
> +		BUG();
> +		break;
> +	}
> +}

PCI_CAP_ID_MSI case seems wrong to me.
I was expecting a readl() in that case as well.

thanks,
grant
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