linux-kernel - Re: [RFC, v1 0/3] msi support for platform devices

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Date:   Tue, 17 Nov 2020 11:55:08 +0530
From:   Vikas Gupta <vikas.gupta@...adcom.com>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Vikram Prakash <vikram.prakash@...adcom.com>,
        Srinath Mannam <srinath.mannam@...adcom.com>,
        Manish Kurup <manish.kurup@...adcom.com>
Subject: Re: [RFC, v1 0/3] msi support for platform devices

Hi Eric,

On Mon, Nov 16, 2020 at 6:44 PM Auger Eric <eric.auger@...hat.com> wrote:
>
> Hi Vikas,
>
> On 11/13/20 6:24 PM, Vikas Gupta wrote:
> > Hi Eric,
> >
> > On Fri, Nov 13, 2020 at 12:10 AM Auger Eric <eric.auger@...hat.com> wrote:
> >>
> >> Hi Vikas,
> >>
> >> On 11/12/20 6:58 PM, Vikas Gupta wrote:
> >>> This RFC adds support for MSI for platform devices.
> >>> a) MSI(s) is/are added in addition to the normal interrupts.
> >>> b) The vendor specific MSI configuration can be done using
> >>>    callbacks which is implemented as msi module.
> >>> c) Adds a msi handling module for the Broadcom platform devices.
> >>>
> >>> Changes from:
> >>> -------------
> >>>  v0 to v1:
> >>>    i)  Removed MSI device flag VFIO_DEVICE_FLAGS_MSI.
> >>>    ii) Add MSI(s) at the end of the irq list of platform IRQs.
> >>>        MSI(s) with first entry of MSI block has count and flag
> >>>        information.
> >>>        IRQ list: Allocation for IRQs + MSIs are allocated as below
> >>>        Example: if there are 'n' IRQs and 'k' MSIs
> >>>        -------------------------------------------------------
> >>>        |IRQ-0|IRQ-1|....|IRQ-n|MSI-0|MSI-1|MSI-2|......|MSI-k|
> >>>        -------------------------------------------------------
> >> I have not taken time yet to look at your series, but to me you should have
> >> |IRQ-0|IRQ-1|....|IRQ-n|MSI|MSIX
> >> then for setting a given MSIX (i) you would select the MSIx index and
> >> then set start=i count=1.
> >
> > As per your suggestion, we should have, if there are n-IRQs, k-MSIXs
> > and m-MSIs, allocation of IRQs should be done as below
> >
> > |IRQ0|IRQ1|......|IRQ-(n-1)|MSI|MSIX|
> >                                              |        |
> >                                              |
> > |MSIX0||MSIX1||MSXI2|....|MSIX-(k-1)|
> >                                              |MSI0||MSI1||MSI2|....|MSI-(m-1)|
> No I really meant this list of indices: IRQ0|IRQ1|......|IRQ-(n-1)|MSI|MSIX|
> and potentially later on IRQ0|IRQ1|......|IRQ-(n-1)|MSI|MSIX| ERR| REQ
> if ERR/REQ were to be added.
I agree on this. Actually the map I drew incorrectly above but wanted
to demonstrate the same. It was a child-parent relationship for MSI
and its members and similarly for MSIX as well.
>
> I think the userspace could query the total number of indices using
> VFIO_DEVICE_GET_INFO and retrieve num_irqs (corresponding to the n wire
> interrupts + MSI index + MSIX index)
>
> Then userspace can loop on all the indices using
> VFIO_DEVICE_GET_IRQ_INFO. For each index it uses count to determine the
> first indices related to wire interrupts (count = 1). Then comes the MSI
> index, and after the MSI index. If any of those is supported, count >1,
> otherwise count=0. The only thing I am dubious about is can the device
> use a single MSI/MSIX? Because my hypothesis here is we use count to
> discriminate between wire first indices and other indices.
I believe count can be one as well, especially for ERR/REQ as you
mentioned above.I think we can not rely on the count > 1. Now, this is
blocking and we are not left with options unless we consider adding
more enums in flags in vfio_irq_info to tell userspace that particular
index is wired, MSI, MSIX etc. for the platform device.
What do you think?
>
>
>
> > With this implementation user space can know that, at indexes n and
> > n+1, edge triggered interrupts are present.
> note wired interrupts can also be edge ones.
> >    We may add an element in vfio_platform_irq itself to allocate MSIs/MSIXs
> >    struct vfio_platform_irq{
> >    .....
> >    .....
> >    struct vfio_platform_irq *block; => this points to the block
> > allocation for MSIs/MSIXs and all msi/msix are type of IRQs.As wired interrupts and MSI interrupts coexist, I would store in vdev an
> array of wired interrupts (the existing vdev->irqs) and a new array for
> MSI(x) as done in the PCI code.
>
> vdev->ctx = kcalloc(nvec, sizeof(struct vfio_pci_irq_ctx), GFP_KERNEL);
>
> Does it make sense?
Yes, we can use similar kinds of allocations.

Thanks,
Vikas
>
> >    };
> >                          OR
> > Another structure can be defined in 'vfio_pci_private.h'
> > struct vfio_msi_ctx {
> >         struct eventfd_ctx      *trigger;
> >         char                    *name;
> > };
> > and
> > struct vfio_platform_irq {
> >   .....
> >   .....
> >   struct vfio_msi_ctx *block; => this points to the block allocation
> > for MSIs/MSIXs
> > };
> > Which of the above two options sounds OK to you? Please suggest.
> >
> >> to me individual MSIs are encoded in the subindex and not in the index.
> >> The index just selects the "type" of interrupt.
> >>
> >> For PCI you just have:
> >>         VFIO_PCI_INTX_IRQ_INDEX,
> >>         VFIO_PCI_MSI_IRQ_INDEX, -> MSI index and then you play with
> >> start/count
> >>         VFIO_PCI_MSIX_IRQ_INDEX,
> >>         VFIO_PCI_ERR_IRQ_INDEX,
> >>         VFIO_PCI_REQ_IRQ_INDEX,
> >>
> >> (include/uapi/linux/vfio.h)
> >
> > In pci case, type of interrupts is fixed so they can be 'indexed' by
> > these enums but for VFIO platform user space will need to iterate all
> > (num_irqs) indexes to know at which indexes edge triggered interrupts
> > are present.
> indeed, but can't you loop over all indices looking until count !=1? At
> this point you know if have finished emurating the wires. Holds if
> MSI(x) count !=1 of course.
>
> Thanks
>
> Eric
>
> >
> > Thanks,
> > Vikas
> >>
> >> Thanks
> >>
> >> Eric
> >>>        MSI-0 will have count=k set and flags set accordingly.
> >>>
> >>> Vikas Gupta (3):
> >>>   vfio/platform: add support for msi
> >>>   vfio/platform: change cleanup order
> >>>   vfio/platform: add Broadcom msi module
> >>>
> >>>  drivers/vfio/platform/Kconfig                 |   1 +
> >>>  drivers/vfio/platform/Makefile                |   1 +
> >>>  drivers/vfio/platform/msi/Kconfig             |   9 +
> >>>  drivers/vfio/platform/msi/Makefile            |   2 +
> >>>  .../vfio/platform/msi/vfio_platform_bcmplt.c  |  74 ++++++
> >>>  drivers/vfio/platform/vfio_platform_common.c  |  86 ++++++-
> >>>  drivers/vfio/platform/vfio_platform_irq.c     | 238 +++++++++++++++++-
> >>>  drivers/vfio/platform/vfio_platform_private.h |  23 ++
> >>>  8 files changed, 419 insertions(+), 15 deletions(-)
> >>>  create mode 100644 drivers/vfio/platform/msi/Kconfig
> >>>  create mode 100644 drivers/vfio/platform/msi/Makefile
> >>>  create mode 100644 drivers/vfio/platform/msi/vfio_platform_bcmplt.c
> >>>
> >>
>

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