linux-kernel - RE: [RFC PATCH 00/42] Sharing KVM TDP to IOMMU

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Date:   Tue, 5 Dec 2023 07:17:31 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Sean Christopherson <seanjc@...gle.com>
CC:     "Zhao, Yan Y" <yan.y.zhao@...el.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>
Subject: RE: [RFC PATCH 00/42] Sharing KVM TDP to IOMMU

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Tuesday, December 5, 2023 3:51 AM
> 
> On Mon, Dec 04, 2023 at 11:22:49AM -0800, Sean Christopherson wrote:
> > It wouldn't even necessarily need to be a notifier per se, e.g. if we taught
> KVM
> > to manage IOMMU page tables, then KVM could simply install mappings for
> multiple
> > sets of page tables as appropriate.

iommu driver still needs to be notified to invalidate the iotlb, unless we want
KVM to directly call IOMMU API instead of going through iommufd.

> 
> This somehow feels more achievable to me since KVM already has all the
> code to handle multiple TDPs, having two parallel ones is probably
> much easier than trying to weld KVM to a different page table
> implementation through some kind of loose coupled notifier.
> 

yes performance-wise this can also reduce the I/O page faults as the
sharing approach achieves.

but how is it compared to another way of supporting IOPF natively in
iommufd and iommu drivers? Note that iommufd also needs to support
native vfio applications e.g. dpdk. I'm not sure whether there will be
strong interest in enabling IOPF for those applications. But if the 
answer is yes then it's inevitable to have such logic implemented in
the iommu stack given KVM is not in the picture there.

With that is it more reasonable to develop the IOPF support natively
in iommu side, plus an optional notifier mechanism to sync with
KVM-induced host PTE installation as optimization?