linux-kernel - Re: [RFC PATCH 15/18] cgroup: Introduce ioasids controller

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Date:   Thu, 4 Mar 2021 11:01:44 -0800
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Tejun Heo <tj@...nel.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>,
        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 15/18] cgroup: Introduce ioasids controller

Hi Jason,

On Thu, 4 Mar 2021 13:54:02 -0400, Jason Gunthorpe <jgg@...dia.com> wrote:

> On Thu, Mar 04, 2021 at 09:46:03AM -0800, Jacob Pan wrote:
> 
> > Right, I was assuming have three use cases of IOASIDs:
> > 1. host supervisor SVA (not a concern, just one init_mm to bind)
> > 2. host user SVA, either one IOASID per process or perhaps some private
> > IOASID for private address space
> > 3. VM use for guest SVA, each IOASID is bound to a guest process
> > 
> > My current cgroup proposal applies to #3 with IOASID_SET_TYPE_MM, which
> > is allocated by the new /dev/ioasid interface.
> > 
> > For #2, I was thinking you can limit the host process via PIDs cgroup?
> > i.e. limit fork. So the host IOASIDs are currently allocated from the
> > system pool with quota of chosen by iommu_sva_init() in my patch, 0
> > means unlimited use whatever is available.
> > https://lkml.org/lkml/2021/2/28/18  
> 
> Why do we need two pools?
> 
> If PASID's are limited then why does it matter how the PASID was
> allocated? Either the thing requesting it is below the limit, or it
> isn't.
> 
you are right. it should be tracked based on the process regardless it is
allocated by the user (/dev/ioasid) or indirectly by kernel drivers during
iommu_sva_bind_device(). Need to consolidate both 2 and 3 and
decouple cgroup and IOASID set.

> For something like qemu I'd expect to put the qemu process in a cgroup
> with 1 PASID. Who cares what qemu uses the PASID for, or how it was
> allocated?
> 
For vSVA, we will need one PASID per guest process. But that is up to the
admin based on whether or how many SVA capable devices are directly
assigned.

> Jason


Thanks,

Jacob