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Date:   Tue, 8 Jun 2021 10:17:56 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     David Gibson <david@gibson.dropbear.id.au>
Cc:     Alex Williamson <alex.williamson@redhat.com>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Jason Wang <jasowang@redhat.com>,
        Eric Auger <eric.auger@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "Liu, Yi L" <yi.l.liu@intel.com>, "Wu, Hao" <hao.wu@intel.com>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Kirti Wankhede <kwankhede@nvidia.com>,
        Robin Murphy <robin.murphy@arm.com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal
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On Tue, Jun 08, 2021 at 12:37:04PM +1000, David Gibson wrote:

> > The PPC/SPAPR support allows KVM to associate a vfio group to an IOMMU
> > page table so that it can handle iotlb programming from pre-registered
> > memory without trapping out to userspace.
> 
> To clarify that's a guest side logical vIOMMU page table which is
> partially managed by KVM.  This is an optimization - things can work
> without it, but it means guest iomap/unmap becomes a hot path because
> each map/unmap hypercall has to go
> 	guest -> KVM -> qemu -> VFIO
> 
> So there are multiple context transitions.

Isn't this overhead true of many of the vIOMMUs? Can the fast path be
generalized?

Jason