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

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Date:   Fri, 19 Mar 2021 10:58:41 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.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>,
        Jason Gunthorpe <jgg@...dia.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>
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs

Hi Jacob,

On Thu, Mar 18, 2021 at 05:22:34PM -0700, Jacob Pan wrote:
> Hi Jean,
> 
> Slightly off the title. As we are moving to use cgroup to limit PASID
> allocations, it would be much simpler if we enforce on the current task.

Yes I think we should do that. Is there a problem with charging the
process that does the PASID allocation even if the PASID indexes some
other mm?

> However, iommu_sva_alloc_pasid() takes an mm_struct pointer as argument
> which implies it can be something other the the current task mm. So far all
> kernel callers use current task mm. Is there a use case for doing PASID
> allocation on behalf of another mm? If not, can we remove the mm argument?

This would effectively remove the mm parameter from
iommu_sva_bind_device(). I'm not opposed to that, but reintroducing it
later will be difficult if IOMMU drivers start assuming that the bound mm
is from current.

Although there is no use for it at the moment (only two upstream users and
it looks like amdkfd always uses current too), I quite like the
client-server model where the privileged process does bind() and programs
the hardware queue on behalf of the client process.

Thanks,
Jean