linux-kernel - Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

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Message-ID: <20210504231530.GE1370958@nvidia.com>
Date:   Tue, 4 May 2021 20:15:30 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        Auger Eric <eric.auger@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        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" <iommu@...ts.linux-foundation.org>,
        "cgroups@...r.kernel.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>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs

On Tue, May 04, 2021 at 03:11:54PM -0700, Jacob Pan wrote:

> > It is a weird way to use xarray to have a structure which
> > itself is just a wrapper around another RCU protected structure.
> > 
> > Make the caller supply the ioasid_data memory, embedded in its own
> > element, get rid of the void * and rely on XA_ZERO_ENTRY to hold
> > allocated but not active entries.
> > 
> Let me try to paraphrase to make sure I understand. Currently
> struct ioasid_data is private to the iasid core, its memory is allocated by
> the ioasid core.
> 
> You are suggesting the following:
> 1. make struct ioasid_data public
> 2. caller allocates memory for ioasid_data, initialize it then pass it to
> ioasid_alloc to store in the xarray
> 3. caller will be responsible for setting private data inside ioasid_data
> and do call_rcu after update if needed.

Basically, but you probably won't need a "private data" once the
caller has this struct as it can just embed it in whatever larger
struct makes sense for it and use container_of/etc

I didn't look too closely at the whole thing though. Honestly I'm a
bit puzzled why we need a pluggable global allocator framework.. The
whole framework went to some trouble to isolate everything into iommu
drivers then that whole design is disturbed by this global thing.

Jason