linux-kernel - Re: [PATCH v4 08/17] iommufd: Always setup MSI and anforce cc on kernel-managed domains

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Message-ID: <20231020135501.GG3952@nvidia.com>
Date:   Fri, 20 Oct 2023 10:55:01 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Nicolin Chen <nicolinc@...dia.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "mjrosato@...ux.ibm.com" <mjrosato@...ux.ibm.com>,
        "chao.p.peng@...ux.intel.com" <chao.p.peng@...ux.intel.com>,
        "yi.y.sun@...ux.intel.com" <yi.y.sun@...ux.intel.com>,
        "peterx@...hat.com" <peterx@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        "lulu@...hat.com" <lulu@...hat.com>,
        "suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "Duan, Zhenzhong" <zhenzhong.duan@...el.com>,
        "Martins, Joao" <joao.m.martins@...cle.com>
Subject: Re: [PATCH v4 08/17] iommufd: Always setup MSI and anforce cc on
 kernel-managed domains

On Fri, Oct 20, 2023 at 02:43:58AM +0000, Tian, Kevin wrote:

> What we want to prevent is attaching a non-CC device to a CC domain
> or upgrade a non-CC domain to CC since in both case the non-CC
> device will be broken due to incompatible page table format.

[..]

> Who cares about such consistency? sure the result is different due to order:
> 
> 1) creating hwpt for dev1 (non-CC) then later attaching hwpt to
>     dev2 (CC) will succeed;
> 
> 2) creating hwpt for dev2 (CC) then later attaching hwpt to
>     dev1 (non-CC) will fail then the user should create a new hwpt
>     for dev1;

AH... So really what the Intel driver wants is not upgrade to CC but
*downgrade* from CC.

non-CC is the type that is universally applicable, so if we come
across a non-CC capable device the proper/optimal thing is to degrade
the HWPT and re-use it, not allocate a new HWPT.

So the whole thing is upside down.

As changing the IOPTEs in flight seems hard, and I don't want to see
the Intel driver get slowed down to accomodate this, I think you are
right to say this should be a creation time property only.

I still think userspace should be able to select it so it can minimize
the number of HWPTs required.

> But the user shouldn't assume such explicit consistency since it's not
> defined in our uAPI. All we defined is that the attaching may
> fail due to incompatibility for whatever reason then the user can
> always try creating a new hwpt for the to-be-attached device. From
> this regard I don't see providing consistency of result is
> necessary. 😊

Anyhow, OK, lets add a comment summarizing your points and remove the
cc upgrade at attach time (sorry Nicolin/Yi!)

It is easy to add a HWPT flag for this later if someone wants to
optimize it.

Jason