Re: [patch 27/33] genirq/msi: Provide constants for PCI/IMS support

[Jason Gunthorpe]

On Fri, Nov 11, 2022 at 02:58:54PM +0100, Thomas Gleixner wrote:
> Provide the necessary constants for PCI/IMS support:
> 
>   - A new bus token for MSI irqdomain identification
>   - A MSI feature flag for the MSI irqdomains to signal support
>   - A secondary domain id
> 
> The latter expands the device internal domain pointer storage array from 1
> to 2 entries. That extra pointer is mostly unused today, but the
> alternative solutions would not be free either and would introduce more
> complexity all over the place. Trade the 8bytes for simplicity.
> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  include/linux/irqdomain_defs.h |    1 +
>  include/linux/msi.h            |    2 ++
>  include/linux/msi_api.h        |    1 +
>  3 files changed, 4 insertions(+)
> 
> --- a/include/linux/irqdomain_defs.h
> +++ b/include/linux/irqdomain_defs.h
> @@ -25,6 +25,7 @@ enum irq_domain_bus_token {
>  	DOMAIN_BUS_PCI_DEVICE_MSIX,
>  	DOMAIN_BUS_DMAR,
>  	DOMAIN_BUS_AMDVI,
> +	DOMAIN_BUS_PCI_DEVICE_IMS,

I don't think we should call this IMS.. GENERIC maybe?

Things that can support IMS should really, IMHO, just not check for
PCI MSI/MSIX and effectively support everything. They don't override
the write_msg, and they don't care how the message is programmed.

> --- a/include/linux/msi.h
> +++ b/include/linux/msi.h
> @@ -525,6 +525,8 @@ enum {
>  	MSI_FLAG_MSIX_CONTIGUOUS	= (1 << 19),
>  	/* PCI/MSI-X vectors can be dynamically allocated/freed post MSI-X enable */
>  	MSI_FLAG_PCI_MSIX_ALLOC_DYN	= (1 << 20),
> +	/* Support for PCI/IMS */
> +	MSI_FLAG_PCI_IMS		= (1 << 21),

Maybe for legacy reasons it is too complicated, but it would be so
much clearer of the special case of "I only know how to support PCI
MSI and PCI MSI-X" was called out as a special flag, and the more
general case of "any write_msg is fine by me" was left behind.

I feel like when the device domain is created in the first place the
parent domain(s) should be able to reject the creation if the
requested child domain is not one it supports. Eg the hypervisor
interactions checks if the child domain is PCI MSI or PCI MSI-X and
rejects otherwise, because that is the only thing the hypervisor knows
how to work with.

If we did that perhaps we don't even need a flag or further checks?

Jason