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

From: Thomas Gleixner
Date: Wed Jul 21 2021 - 15:28:29 EST

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:

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@xxxxxxxxx>
Fixes: f036d4ea5fa7 ("[PATCH] ia32 Message Signalled Interrupt support")
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
Cc: linux-pci@xxxxxxxxxxxxxxx
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);
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;