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

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Date:   Mon, 29 Nov 2021 21:51:10 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Logan Gunthorpe <logang@...tatee.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, Marc Zygnier <maz@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.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>
Subject: Re: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()

Logan,

On Mon, Nov 29 2021 at 11:21, Logan Gunthorpe wrote:
> On 2021-11-26 6:23 p.m., Thomas Gleixner wrote:
>> Replace the about to vanish iterators, make use of the filtering and take
>> the descriptor lock around the iteration.
>
> This patch looks good to me:
>
> Reviewed-by: Logan Gunthorpe <logang@...tatee.com>

thanks for having a look at this. While I have your attention, I have a
question related to NTB.

The switchtec driver is the only one which uses PCI_IRQ_VIRTUAL in order
to allocate non-hardware backed MSI-X descriptors.

AFAIU these descriptors are not MSI-X descriptors in the regular sense
of PCI/MSI-X. They are allocated via the PCI/MSI mechanism but their
usage is somewhere in NTB which has nothing to do with the way how the
real MSI-X interrupts of a device work which explains why we have those
pci.msi_attrib.is_virtual checks all over the place.

I assume that there are other variants feeding into NTB which can handle
that without this PCI_IRQ_VIRTUAL quirk, but TBH, I got completely lost
in that code.

Could you please shed some light on the larger picture of this?

Thanks,

        tglx