linux-kernel - Re: [patch 21/32] NTB/msi: Convert to msi_on_each

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Date:   Tue, 30 Nov 2021 20:17:48 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Marc Zygnier <maz@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Megha Dey <megha.dey@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-pci@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jon Mason <jdmason@...zu.us>,
        Dave Jiang <dave.jiang@...el.com>,
        Allen Hubbe <allenbh@...il.com>, linux-ntb@...glegroups.com,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>, x86@...nel.org
Subject: Re: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()

On Tue, Nov 30, 2021 at 10:23:16PM +0100, Thomas Gleixner wrote:

> > If this doesn't become an irq_chip what other way is there to properly
> > program the addr/data pair as drivers/ntb/msi.c is doing?
> 
> That's not the question. This surely will be a separate irq chip and a
> separate irqdomain.

OK

> The real problem is where to store the MSI descriptors because the PCI
> device has its own real PCI/MSI-X interrupts which means it still shares
> the storage space.

Er.. I never realized that just looking at the patches :|

That is relevant to all real "IMS" users. IDXD escaped this because
it, IMHO, wrongly used the mdev with the IRQ layer. The mdev is purely
a messy artifact of VFIO, it should not be required to make the IRQ
layers work.

I don't think it makes sense that the msi_desc would point to a mdev,
the iommu layer consumes the msi_desc_to_dev(), it really should point
to the physical device that originates the message with a proper
iommu ops/data/etc.

> I'm currently tending to partition the index space in the xarray:
> 
>  0x00000000 - 0x0000ffff          PCI/MSI-X
>  0x00010000 - 0x0001ffff          NTB

It is OK, with some xarray work it can be range allocating & reserving
so that the msi_domain_alloc_irqs() flows can carve out chunks of the
number space..

Another view is the msi_domain_alloc_irqs() flows should have their
own xarrays..

> which is feasible now with the range modifications and way simpler to do
> with xarray than with the linked list.

Indeed!

Regards,
Jason