Re: [PATCH v2] PCI/MSI: Avoid torn updates to MSI pairs

[Rajat Jain <rajatja@google.com>]

On Fri, Jan 17, 2020 at 4:26 PM Evan Green <evgreen@chromium.org> wrote:
>
> __pci_write_msi_msg() updates three registers in the device: address
> high, address low, and data. On x86 systems, address low contains
> CPU targeting info, and data contains the vector. The order of writes
> is address, then data.
>
> This is problematic if an interrupt comes in after address has
> been written, but before data is updated, and both the SMP affinity
> and target vector are being changed. In this case, the interrupt targets
> the wrong vector on the new CPU.
>
> This case is pretty easy to stumble into using xhci and CPU hotplugging.
> Create a script that repeatedly targets interrupts at a set of cores and
> then offlines those cores. Put some stress on USB, and then watch xhci
> lose an interrupt and die.

Do I understand it right, that even with this patch, the driver might
still miss the same interrupt (because we are disabling the interrupt
for that time) -  the improvement this patch brings is that it will at
least not be delivered to the wrong CPU or via a wrong vector?

Thanks,
Rajat

>
> Avoid this by disabling MSIs during the update.
>
> Signed-off-by: Evan Green <evgreen@chromium.org>
> ---
>
> Changes in v2:
> - Also mask msi-x interrupts during the update
> - Restore the enable/mask bit to its previous value, rather than
> unconditionally enabling interrupts
>
>
> Bjorn,
> I was unsure whether disabling MSIs temporarily is actually an okay
> thing to do. I considered using the mask bit, but got the impression
> that not all devices support the mask bit. Let me know if this going to
> cause problems or there's a better way. I can include the repro
> script I used to cause mayhem if needed.
>
> ---
>  drivers/pci/msi.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> index 6b43a5455c7af..bb21a7739fa2c 100644
> --- a/drivers/pci/msi.c
> +++ b/drivers/pci/msi.c
> @@ -311,6 +311,7 @@ void __pci_read_msi_msg(struct msi_desc *entry, struct msi_msg *msg)
>  void __pci_write_msi_msg(struct msi_desc *entry, struct msi_msg *msg)
>  {
>         struct pci_dev *dev = msi_desc_to_pci_dev(entry);
> +       u16 msgctl;
>
>         if (dev->current_state != PCI_D0 || pci_dev_is_disconnected(dev)) {
>                 /* Don't touch the hardware now */
> @@ -320,15 +321,25 @@ void __pci_write_msi_msg(struct msi_desc *entry, struct msi_msg *msg)
>                 if (!base)
>                         goto skip;
>
> +               pci_read_config_word(dev, dev->msix_cap + PCI_MSIX_FLAGS,
> +                                    &msgctl);
> +
> +               pci_write_config_word(dev, dev->msix_cap + PCI_MSIX_FLAGS,
> +                                     msgctl | PCI_MSIX_FLAGS_MASKALL);
> +
>                 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);
> +               pci_write_config_word(dev, dev->msix_cap + PCI_MSIX_FLAGS,
> +                                     msgctl);
> +
>         } else {
>                 int pos = dev->msi_cap;
> -               u16 msgctl;
> +               u16 enabled;
>
>                 pci_read_config_word(dev, pos + PCI_MSI_FLAGS, &msgctl);
> -               msgctl &= ~PCI_MSI_FLAGS_QSIZE;
> +               enabled = msgctl & PCI_MSI_FLAGS_ENABLE;
> +               msgctl &= ~(PCI_MSI_FLAGS_QSIZE | PCI_MSI_FLAGS_ENABLE);
>                 msgctl |= entry->msi_attrib.multiple << 4;
>                 pci_write_config_word(dev, pos + PCI_MSI_FLAGS, msgctl);
>
> @@ -343,6 +354,9 @@ void __pci_write_msi_msg(struct msi_desc *entry, struct msi_msg *msg)
>                         pci_write_config_word(dev, pos + PCI_MSI_DATA_32,
>                                               msg->data);
>                 }
> +
> +               msgctl |= enabled;
> +               pci_write_config_word(dev, pos + PCI_MSI_FLAGS, msgctl);
>         }
>
>  skip:
> --
> 2.24.1
>