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

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Date:   Wed, 24 Mar 2021 15:12:30 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     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, 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: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs

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;
	}

BTW, IOASID will be one of the resources under the proposed misc cgroup.

Thanks,

Jacob