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

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Message-ID: <ZTWabb6AbOTFNgaw@Asurada-Nvidia>
Date:   Sun, 22 Oct 2023 17:18:03 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>
CC:     "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 Sat, Oct 21, 2023 at 01:38:04PM -0300, Jason Gunthorpe wrote:
> On Fri, Oct 20, 2023 at 11:59:13AM -0700, Nicolin Chen wrote:
> > On Fri, Oct 20, 2023 at 10:55:01AM -0300, Jason Gunthorpe wrote:
> > > 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!)
> > 
> > Ack. I will send a small removal series. I assume it should CC
> > stable tree also? 
> 
> No, it seems more like tidying that fixing a functional issue, do I
> misunderstand?

Hmm. Maybe the misunderstanding is mine -- Kevin was asking if
it was already a bug and you answered yes:
https://lore.kernel.org/linux-iommu/20231016115736.GP3952@nvidia.com/

If this shouldn't be a bug fix, I could just merge them into a
single tidying patch and add the comments you suggested below.

> > And where should we add this comment? Kdoc of
> > the alloc uAPI?
> 
> Maybe right in front of the only enforce_cc op callback?

OK. How about this? Might be a bit verbose though:
	/*
	 * enforce_cache_coherenc must be determined during the HWPT allocation.
	 * Note that a HWPT (non-CC) created for a device (non-CC) can be later
	 * reused by another device (either non-CC or CC). However, A HWPT (CC)
	 * created for a device (CC) cannot be reused by another device (non-CC)
	 * but only devices (CC). Instead user space in this case would need to
	 * allocate a separate HWPT (non-CC).
	 */

Thanks
Nic