linux-kernel - Re: [RFC v2 2/2] cgroup: sev: Miscellaneous cgroup documentation.

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Date:   Mon, 15 Mar 2021 18:19:35 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Jacob Pan <jacob.jun.pan@...el.com>
Cc:     Vipin Sharma <vipinsh@...gle.com>, mkoutny@...e.com,
        rdunlap@...radead.org, thomas.lendacky@....com,
        brijesh.singh@....com, jon.grimm@....com, eric.vantassell@....com,
        pbonzini@...hat.com, hannes@...xchg.org, frankja@...ux.ibm.com,
        borntraeger@...ibm.com, corbet@....net, seanjc@...gle.com,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, gingell@...gle.com,
        rientjes@...gle.com, dionnaglaze@...gle.com, kvm@...r.kernel.org,
        x86@...nel.org, cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, "Tian, Kevin" <kevin.tian@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>
Subject: Re: [RFC v2 2/2] cgroup: sev: Miscellaneous cgroup documentation.

Hello,

On Mon, Mar 15, 2021 at 03:11:55PM -0700, Jacob Pan wrote:
> > Migration itself doesn't have restrictions but all resources are
> > distributed on the same hierarchy, so the controllers are supposed to
> > follow the same conventions that can be implemented by all controllers.
> > 
> Got it, I guess that is the behavior required by the unified hierarchy.
> Cgroup v1 would be ok? But I am guessing we are not extending on v1?

A new cgroup1 only controller is unlikely to be accpeted.

> The IOASIDs are programmed into devices to generate DMA requests tagged
> with them. The IOMMU has a per device IOASID table with each entry has two
> pointers:
>  - the PGD of the guest process.
>  - the PGD of the host process
> 
> The result of this 2 stage/nested translation is that we can share virtual
> address (SVA) between guest process and DMA. The host process needs to
> allocate multiple IOASIDs since one IOASID is needed for each guest process
> who wants SVA.
> 
> The DMA binding among device-IOMMU-process is setup via a series of user
> APIs (e.g. via VFIO).
> 
> If a process calls fork(), the children does not inherit the IOASIDs and
> their bindings. Children who wish to use SVA has to call those APIs to
> establish the binding for themselves.
> 
> Therefore, if a host process allocates 10 IOASIDs then does a
> fork()/clone(), it cannot charge 10 IOASIDs in the new cgroup. i.e. the 10
> IOASIDs stays with the process wherever it goes.
> 
> I feel this fit in the domain model, true?

I still don't get where migration is coming into the picture. Who's
migrating where?

Thanks.

-- 
tejun