linux-kernel - [patch 4/8] PCI/MSI: Enforce MSI[X] entry updates to be visible

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Message-Id: <20210721192650.499597025@linutronix.de>
Date:   Wed, 21 Jul 2021 21:11:30 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Kevin Tian <kevin.tian@...el.com>,
        Marc Zyngier <maz@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        x86@...nel.org
Subject: [patch 4/8] PCI/MSI: Enforce MSI[X] entry updates to be visible

Nothing enforces the posted writes to be visible when the function
returns. Flush them even if the flush might be redundant when the entry is
masked already as the unmask will flush as well. This is either setup or a
rare affinity change event so the extra flush is not the end of the world.

While this is more a theoretical issue especially the X86 MSI affinity
stter mechanism relies on the assumption that the update has reached the
hardware when the function returns.

Again, as this never has been enfocred the Fixes tag refers to a commit in:
   git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git

Fixes: f036d4ea5fa7 ("[PATCH] ia32 Message Signalled Interrupt support")
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>
Cc: linux-pci@...r.kernel.org
---
 drivers/pci/msi.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/drivers/pci/msi.c
+++ b/drivers/pci/msi.c
@@ -311,6 +311,9 @@ void __pci_write_msi_msg(struct msi_desc
 
 		if (unmasked)
 			__pci_msix_desc_mask_irq(entry, 0);
+
+		/* Ensure that the writes are visible in the device */
+		readl(base + PCI_MSIX_ENTRY_DATA);
 	} else {
 		int pos = dev->msi_cap;
 		u16 msgctl;
@@ -331,6 +334,8 @@ void __pci_write_msi_msg(struct msi_desc
 			pci_write_config_word(dev, pos + PCI_MSI_DATA_32,
 					      msg->data);
 		}
+		/* Ensure that the writes are visible in the device */
+		pci_read_config_word(dev, pos + PCI_MSI_FLAGS, &msgctl);
 	}
 
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