linux-kernel - Re: [RFC V1 RESEND 0/6] Introduce dynamic allocation/freeing of MSI-X vectors

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Date:   Thu, 1 Aug 2019 19:24:42 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Megha Dey <megha.dey@...ux.intel.com>
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, marc.zyngier@....com, ashok.raj@...el.com,
        jacob.jun.pan@...ux.intel.com, megha.dey@...el.com
Subject: Re: [RFC V1 RESEND 0/6] Introduce dynamic allocation/freeing of
 MSI-X vectors

Hi Megha,

On Fri, Jun 21, 2019 at 05:19:32PM -0700, Megha Dey wrote:
> Currently, MSI-X vector enabling and allocation for a PCIe device is
> static i.e. a device driver gets only one chance to enable a specific
> number of MSI-X vectors, usually during device probe. Also, in many
> cases, drivers usually reserve more than required number of vectors
> anticipating their use, which unnecessarily blocks resources that
> could have been made available to other devices. Lastly, there is no
> way for drivers to reserve more vectors, if the MSI-x has already been
> enabled for that device.
>  
> Hence, a dynamic MSI-X kernel infrastructure can benefit drivers by
> deferring MSI-X allocation to post probe phase, where actual demand
> information is available.
>  
> This patchset enables the dynamic allocation/de-allocation of MSI-X
> vectors by introducing 2 new APIs:
> pci_alloc_irq_vectors_dyn() and pci_free_irq_vectors_grp():
> 
> We have had requests from some of the NIC/RDMA users who have lots of
> interrupt resources and would like to allocate them on demand,
> instead of using an all or none approach.
> 
> The APIs are fairly well tested (multiple allocations/deallocations),
> but we have no early adopters yet. Hence, sending this series as an
> RFC for review and comments.
> 
> The patches are based out of Linux 5.2-rc5.
> 
> Megha Dey (6):
>   PCI/MSI: New structures/macros for dynamic MSI-X allocation
>   PCI/MSI: Dynamic allocation of MSI-X vectors by group
>   x86: Introduce the dynamic teardown function
>   PCI/MSI: Introduce new structure to manage MSI-x entries

s/MSI-x/MSI-X/
If "entries" here means the same as "vectors" above, please use the
same word.

>   PCI/MSI: Free MSI-X resources by group

Is "resources" the same as "vectors"?

>   Documentation: PCI/MSI: Document dynamic MSI-X infrastructure

When you post a v2 after addressing Thomas' comments, please make
these subject lines imperative sentences beginning with a descriptive
verb.  You can run "git log --oneline drivers/pci" to see the style.
If you're adding a specific interface or structure, mention it by name
in the subject if it's practical.  The "x86" line needs a little more
context; I assume it should include "IRQ", "MSI-X", or something.

>  Documentation/PCI/MSI-HOWTO.txt |  38 +++++
>  arch/x86/include/asm/x86_init.h |   1 +
>  arch/x86/kernel/x86_init.c      |   6 +
>  drivers/pci/msi.c               | 363 +++++++++++++++++++++++++++++++++++++---
>  drivers/pci/probe.c             |   9 +
>  include/linux/device.h          |   3 +
>  include/linux/msi.h             |  13 ++
>  include/linux/pci.h             |  61 +++++++
>  kernel/irq/msi.c                |  34 +++-
>  9 files changed, 497 insertions(+), 31 deletions(-)
> 
> -- 
> 2.7.4
>