linux-kernel - Re: [PATCH v7 1/3] iommufd: Add data structure for Intel VT-d stage-1 cache invalidation

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Date:   Fri, 24 Nov 2023 09:46:45 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.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>,
        "nicolinc@...dia.com" <nicolinc@...dia.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>,
        "joao.m.martins@...cle.com" <joao.m.martins@...cle.com>,
        "Zeng, Xin" <xin.zeng@...el.com>,
        "Zhao, Yan Y" <yan.y.zhao@...el.com>
Subject: Re: [PATCH v7 1/3] iommufd: Add data structure for Intel VT-d
 stage-1 cache invalidation

On Fri, Nov 24, 2023 at 03:00:45AM +0000, Tian, Kevin wrote:

> > I'm fully expecting that Intel will adopt an direct-DMA flush queue
> > like SMMU and AMD have already done as a performance optimization. In
> > this world it makes no sense that the behavior of the direct DMA queue
> > and driver mediated queue would be different.
> > 
> 
> that's a orthogonal topic. I don't think the value of direct-DMA flush
> queue should prevent possible optimization in the mediation path
> (as long as guest-expected deterministic behavior is sustained).

Okay, well as long as the guest is generating the ATC invalidations we
can always make an iommufd API flag to include or exclude the ATC
invalidation when doing the ASID invalidation. So we aren't trapped

Jason