Re: [dpdk-dev] [RFC PATCH] bus/pci: avoid depending on private value in kernel source

[Tiwei Bie <tiwei.bie@intel.com>]

On Wed, Jul 03, 2019 at 10:26:39AM +0200, David Marchand wrote:
> On Wed, Jul 3, 2019 at 10:17 AM Tiwei Bie <tiwei.bie@intel.com> wrote:
> 
>     On Wed, Jul 03, 2019 at 10:01:44AM +0200, David Marchand wrote:
>     > On Wed, Jul 3, 2019 at 9:58 AM Tiwei Bie <tiwei.bie@intel.com> wrote:
>     >
>     >     On Wed, Jul 03, 2019 at 09:36:26AM +0200, David Marchand wrote:
>     >     > On Wed, Jul 3, 2019 at 9:35 AM Tiwei Bie <tiwei.bie@intel.com>
>     wrote:
>     >     >
>     >     >     Hi David,
>     >     >
>     >     >     On Wed, Jul 03, 2019 at 09:02:59AM +0200, David Marchand wrote:
>     >     >     > Hello,
>     >     >     >
>     >     >     > On Wed, Jul 3, 2019 at 7:47 AM Tiwei Bie <tiwei.bie@intel.com
>     >
>     >     wrote:
>     >     >     >
>     >     >     >     The value 40 used in VFIO_GET_REGION_ADDR() is a private
>     value
>     >     >     >     (VFIO_PCI_OFFSET_SHIFT) defined in Linux kernel source
>     [1]. It
>     >     >     >     is not part of VFIO API, and we should not depend on it.
>     >     >     >
>     >     >     >     [1] https://github.com/torvalds/linux/blob/6fbc7275c7a9/
>     drivers
>     >     /vfio/
>     >     >     pci/
>     >     >     >     vfio_pci_private.h#L19
>     >     >     >
>     >     >     >
>     >     >     >
>     >     >     > I did not follow linux kernel changes, is there something
>     that
>     >     would
>     >     >     change
>     >     >     > this offset?
>     >     >     > It looks like a cleanup (did not look into the details yet),
>     do we
>     >     need
>     >     >     this
>     >     >     > now?
>     >     >
>     >     >     In VFIO/mdev [1], the offset can be something different. It
>     depends
>     >     >     on the parent device. It's not just a cleanup. It's a
>     preparation
>     >     >     for the mdev support in DPDK.
>     >     >
>     >     >     [1] https://github.com/torvalds/linux/blob/master/Documentation
>     /
>     >     >     vfio-mediated-device.txt
>     >     >
>     >     >
>     >     >
>     >     > Ok, thanks.
>     >     > So we can wait for mdev to be ready before working on this.
>     >
>     >     What do you mean by "mdev to be ready"? RFC ready? I don't see
>     >     anything blocking the discussion on this now.
>     >
>     >     PS. I already sent a RFC series of the mdev support in DPDK
>     >         to the mailing list 3 month ago.
>     >
>     >
>     > If you need it and the mdev support has been posted already, why not send
>     a n+1
>     > patchset with this patch in it?
>     >
>     > This patch alone looked odd to me.
> 
>     That series was using the old API which assumes the shift
>     is 40 which may not work in some cases. And this patch is
>     to fix the API. I think this patch is actually trying to
>     fix a relatively independent issue -- i.e. switching to using
>     the proper VFIO API to get the region offsets instead of
>     depending on kernel code's internal value.
> 
> 
> 
> Fix, then there is something broken ?

I should use "fix" (with "") actually :)

> You said this is for mdev support which is not currently part of the features
> supported by DPDK.
> 
> 
> This patch breaks the ABI by extending rte_pci_device.
> You must rework it to avoid this break.

I didn't expect it to be merged in this release. I just want
to draw other's attention on this and kick off the discussion
(it would be great if you would like to share your thoughts
on this). If there is a way to avoid extending rte_pci_device,
it would be definitely great. But if we have to break it, then
we would want to send out the announce as early as possible.

Thanks,
Tiwei

> 
> 
> --
> David Marchand