Re: [patch 3/8] PCI/MSI: Enforce that MSI-X table entry is masked for update

Raj, Ashok
2021-07-21T22:33:08Z

On Wed, Jul 21, 2021 at 09:11:29PM +0200, Thomas Gleixner wrote:
> The specification states:
>
> For MSI-X, a function is permitted to cache Address and Data values
> from unmasked MSI-X Table entries. However, anytime software unmasks a
> currently masked MSI-X Table entry either by clearing its Mask bit or
> by clearing the Function Mask bit, the function must update any Address
> or Data values that it cached from that entry. If software changes the
> Address or Data value of an entry while the entry is unmasked, the
> result is undefined.
>
> The Linux kernel's MSI-X support never enforced that the entry is masked
> before the entry is modified hence the Fixes tag refers to a commit in:
> git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
>
> Enforce the entry to be masked across the update.
>
> There is no point in enforcing this to be handled at all possible call
> sites as this is just pointless code duplication and the common update
> function is the obvious place to enforce this.
>
> Reported-by: Kevin Tian <kevin.tian@intel.com>
> Fixes: f036d4ea5fa7 ("[PATCH] ia32 Message Signalled Interrupt support")
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Bjorn Helgaas <bhelgaas@google.com>
> Cc: linux-pci@vger.kernel.org
> ---
> drivers/pci/msi.c | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
> --- a/drivers/pci/msi.c
> +++ b/drivers/pci/msi.c
> @@ -289,13 +289,28 @@ void __pci_write_msi_msg(struct msi_desc
> /* Don't touch the hardware now */
> } else if (entry->msi_attrib.is_msix) {
> void __iomem *base = pci_msix_desc_addr(entry);
> + bool unmasked = !(entry->masked & PCI_MSIX_ENTRY_CTRL_MASKBIT);
>
> if (!base)
> goto skip;
>
> + /*
> + * The specification mandates that the entry is masked
> + * when the message is modified:
> + *
> + * "If software changes the Address or Data value of an
> + * entry while the entry is unmasked, the result is
> + * undefined."
> + */
> + if (unmasked)
> + __pci_msix_desc_mask_irq(entry, PCI_MSIX_ENTRY_CTRL_MASKBIT);
> +

Is there any locking needs here? say during cpu hotplug and some user-space
setting affinity?

> writel(msg->address_lo, base + PCI_MSIX_ENTRY_LOWER_ADDR);
> writel(msg->address_hi, base + PCI_MSIX_ENTRY_UPPER_ADDR);
> writel(msg->data, base + PCI_MSIX_ENTRY_DATA);
> +
> + if (unmasked)
> + __pci_msix_desc_mask_irq(entry, 0);
> } else {
> int pos = dev->msi_cap;
> u16 msgctl;
>