linux-kernel - Re: [RFC PATCH 18/18] ioasid: Add /dev/ioasid for userspace

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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