Re: [PATCH v11 07/16] genirq/msi: Cache MSI message in irq_chip_write_msi_msg()

[Thomas Gleixner <tglx@kernel.org>]

On Tue, Sep 29 2026 at 03:45, Pranjal Shrivastava wrote:
> Currently, PCI MSI caches the composed MSI message into the MSI
> descriptor inside __pci_write_msi_msg(), whereas platform MSI and other
> device MSI domains do not cache the message in the core, forcing
> individual drivers to manually copy the message into desc->msg if they
> need to restore it across power transitions.
>
> Cache desc->msg in irq_chip_write_msi_msg() before invoking the chip
> callback so all hierarchical MSI domains automatically record the
> latest message. Document why the redundant assignment in
> __pci_write_msi_msg() must remain for direct callers outside the
> irq_chip hierarchy.
>
> Suggested-by: Thomas Gleixner <tglx@kernel.org>
> Signed-off-by: Pranjal Shrivastava <praan@google.com>
> ---
>  drivers/pci/msi/msi.c | 6 ++++++
>  kernel/irq/msi.c      | 9 +++++++++
>  2 files changed, 15 insertions(+)
>
> diff --git a/drivers/pci/msi/msi.c b/drivers/pci/msi/msi.c
> index 80a9db417dc8..f7bedc31d486 100644
> --- a/drivers/pci/msi/msi.c
> +++ b/drivers/pci/msi/msi.c
> @@ -257,6 +257,12 @@ void __pci_write_msi_msg(struct msi_desc *entry, struct msi_msg *msg)
>  		pci_write_msg_msi(dev, entry, msg);
>  	}
>  
> +	/*
> +	 * Although irq_chip_write_msi_msg() already caches the message,
> +	 * keep the assignment here for callers that invoke
> +	 * __pci_write_msi_msg() or pci_write_msi_msg() directly outside
> +	 * the irq_chip hierarchy.
> +	 */
>  	entry->msg = *msg;
>  
>  	if (entry->write_msi_msg)
> diff --git a/kernel/irq/msi.c b/kernel/irq/msi.c
> index fb5f372215bf..df460a574d80 100644
> --- a/kernel/irq/msi.c
> +++ b/kernel/irq/msi.c
> @@ -634,6 +634,15 @@ static unsigned int msi_domain_get_hwsize(struct device *dev, unsigned int domid
>  static inline void irq_chip_write_msi_msg(struct irq_data *data,
>  					  struct msi_msg *msg)
>  {
> +	struct msi_desc *desc = irq_data_get_msi_desc(data);
> +
> +	/*
> +	 * PCI Multi-MSI shares a single descriptor across all vectors and
> +	 * the device stores only the base message. Cache the message of the
> +	 * first interrupt only, so that sub-vectors cannot overwrite it.
> +	 */
> +	if (desc->irq == data->irq)
> +		desc->msg = *msg;
>  	data->chip->irq_write_msi_msg(data, msg);
>  }

Sashiko reported correctly:

> [Severity: Critical]
> Does this code cause a NULL pointer dereference for domains without an
> msi_desc object?

So this needs to be:

	if (desc && desc->irq == data->irq)
		desc->msg = *msg;

Did not think about that, but that also means that this patch was not
tested on any recent (less than 10 years old) Intel x86 platform or a
x86 VM exposing HPET...

Thanks,

        tglx