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

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Date:   Thu, 25 Mar 2021 11:21:40 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        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>
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs

On Wed, Mar 24, 2021 at 03:12:30PM -0700, Jacob Pan wrote:
> Hi Jason,
> 
> On Wed, 24 Mar 2021 14:03:38 -0300, Jason Gunthorpe <jgg@...dia.com> wrote:
> 
> > On Wed, Mar 24, 2021 at 10:02:46AM -0700, Jacob Pan wrote:
> > > > Also wondering about device driver allocating auxiliary domains for
> > > > their private use, to do iommu_map/unmap on private PASIDs (a clean
> > > > replacement to super SVA, for example). Would that go through the
> > > > same path as /dev/ioasid and use the cgroup of current task?  
> > >
> > > For the in-kernel private use, I don't think we should restrict based on
> > > cgroup, since there is no affinity to user processes. I also think the
> > > PASID allocation should just use kernel API instead of /dev/ioasid. Why
> > > would user space need to know the actual PASID # for device private
> > > domains? Maybe I missed your idea?  
> > 
> > There is not much in the kernel that isn't triggered by a process, I
> > would be careful about the idea that there is a class of users that
> > can consume a cgroup controlled resource without being inside the
> > cgroup.
> > 
> > We've got into trouble before overlooking this and with something
> > greenfield like PASID it would be best built in to the API to prevent
> > a mistake. eg accepting a cgroup or process input to the allocator.
> > 
> Make sense. But I think we only allow charging the current cgroup, how about
> I add the following to ioasid_alloc():
> 
> 	misc_cg = get_current_misc_cg();
> 	ret = misc_cg_try_charge(MISC_CG_RES_IOASID, misc_cg, 1);
> 	if (ret) {
> 		put_misc_cg(misc_cg);
> 		return ret;
> 	}

Does that allow PASID allocation during driver probe, in kernel_init or
modprobe context?

Thanks,
Jean