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Message-ID: <20210311145534.6fe9bb9a@jacob-builder>
Date: Thu, 11 Mar 2021 14:55:34 -0800
From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
Joerg Roedel <joro@...tes.org>,
Lu Baolu <baolu.lu@...ux.intel.com>,
David Woodhouse <dwmw2@...radead.org>,
iommu@...ts.linux-foundation.org, cgroups@...r.kernel.org,
Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
Johannes Weiner <hannes@...xchg.org>,
Jean-Philippe Brucker <jean-philippe@...aro.com>,
Alex Williamson <alex.williamson@...hat.com>,
Eric Auger <eric.auger@...hat.com>,
Jonathan Corbet <corbet@....net>,
Raj Ashok <ashok.raj@...el.com>,
"Tian, Kevin" <kevin.tian@...el.com>, Yi Liu <yi.l.liu@...el.com>,
Wu Hao <hao.wu@...el.com>, Dave Jiang <dave.jiang@...el.com>,
jacob.jun.pan@...ux.intel.com
Subject: Re: [RFC PATCH 18/18] ioasid: Add /dev/ioasid for userspace
Hi Jason,
Thanks for the review.
On Wed, 10 Mar 2021 15:23:01 -0400, Jason Gunthorpe <jgg@...dia.com> wrote:
> On Sat, Feb 27, 2021 at 02:01:26PM -0800, Jacob Pan wrote:
>
> > +/* -------- IOCTLs for IOASID file descriptor (/dev/ioasid) -------- */
> > +
> > +/**
> > + * IOASID_GET_API_VERSION - _IO(IOASID_TYPE, IOASID_BASE + 0)
> > + *
> > + * Report the version of the IOASID API. This allows us to bump the
> > entire
> > + * API version should we later need to add or change features in
> > incompatible
> > + * ways.
> > + * Return: IOASID_API_VERSION
> > + * Availability: Always
> > + */
> > +#define IOASID_GET_API_VERSION _IO(IOASID_TYPE,
> > IOASID_BASE + 0)
>
> I think this is generally a bad idea, if you change the API later then
> also change the ioctl numbers and everything should work out
>
> eg use the 4th argument to IOC to specify something about the ABI
>
Let me try to understand the idea, do you mean something like this?
#define IOASID_GET_INFO _IOC(_IOC_NONE, IOASID_TYPE, IOASID_BASE + 1,
sizeof(struct ioasid_info))
If we later change the size of struct ioasid_info, IOASID_GET_INFO would be
a different ioctl number. Then we will break the existing user space that
uses the old number. So I am guessing you meant we need to have a different
name also. i.e.
#define IOASID_GET_INFO_V2 _IOC(_IOC_NONE, IOASID_TYPE, IOASID_BASE + 1,
sizeof(struct ioasid_info_v2))
We can get rid of the API version, just have individual IOCTL version.
Is that right?
> Jason
Thanks,
Jacob
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