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

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Message-ID: <ZS7nb+mKanGFXhZY@Asurada-Nvidia>
Date:   Tue, 17 Oct 2023 12:58:39 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     "Tian, Kevin" <kevin.tian@...el.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 Tue, Oct 17, 2023 at 12:53:01PM -0300, Jason Gunthorpe wrote:
> On Tue, Oct 17, 2023 at 08:52:49AM +0000, Tian, Kevin wrote:
> > I prefer to removing enforce_cc in attach fn completely then no parent
> > trick in this patch. Just keep it in hwpt_alloc and leave to iommu driver to
> > figure out the attaching compatibility:
> 
> You are basically saying to set the cc mode during creation because we
> know the idev at that moment and can tell if it should be on/off?
> 
> It seems reasonable, but I can't remember why it is in the attach path
> at the moment.

This was the commit adding it in the alloc path:
https://lore.kernel.org/linux-iommu/8e897628-61fa-b3fb-b609-44eeda11b45e@arm.com/

The older code was doing a hwpt "upgrade" from !cc to cc:
-       /*
-        * Try to upgrade the domain we have, it is an iommu driver bug to
-        * report IOMMU_CAP_ENFORCE_CACHE_COHERENCY but fail
-        * enforce_cache_coherency when there are no devices attached to the
-        * domain.
-        */
-       if (idev->enforce_cache_coherency && !hwpt->enforce_cache_coherency) {
-               if (hwpt->domain->ops->enforce_cache_coherency)
-                       hwpt->enforce_cache_coherency =
-                               hwpt->domain->ops->enforce_cache_coherency(
-                                       hwpt->domain);

If we remove the enforce_cc call in the attach path and let the
driver decide whether to enforce or reject in attach_dev calls,
there seems to be no point in tracking an enforce_cache_coherency
flag in the IOMMUFD pathway but only for the VFIO_DMA_CC_IOMMU
extension check in the vfio-compat pathway?

Thanks
Nic