linux-kernel - RE: [PATCH v7 11/11] iommu/vt-d: Add svm/sva invalidate function

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Message-ID: <AADFC41AFE54684AB9EE6CBC0274A5D19D5DF155@SHSMSX104.ccr.corp.intel.com>
Date:   Tue, 29 Oct 2019 18:52:01 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
CC:     Lu Baolu <baolu.lu@...ux.intel.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        "Alex Williamson" <alex.williamson@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Jonathan Cameron" <jic23@...nel.org>,
        Eric Auger <eric.auger@...hat.com>
Subject: RE: [PATCH v7 11/11] iommu/vt-d: Add svm/sva invalidate function

> From: Jacob Pan [mailto:jacob.jun.pan@...ux.intel.com]
> Sent: Tuesday, October 29, 2019 12:11 AM
> 
> On Mon, 28 Oct 2019 06:06:33 +0000
> "Tian, Kevin" <kevin.tian@...el.com> wrote:
> 
> > > >>> +    /* PASID based dev TLBs, only support all PASIDs or single
> > > >>> PASID */
> > > >>> +    {1, 1, 0},
> > > >>
> > > >> I forgot previous discussion. is it necessary to pass down dev
> > > >> TLB invalidation
> > > >> requests? Can it be handled by host iOMMU driver automatically?
> > > >
> > > > On host SVA, when a memory is unmapped, driver callback will
> > > > invalidate dev IOTLB explicitly. So I guess we need to pass down
> > > > it for guest case. This is also required for guest iova over 1st
> > > > level usage as far as can see.
> > > >
> > >
> > > Sorry, I confused guest vIOVA and guest vSVA. For guest vIOVA, no
> > > device TLB invalidation pass down. But currently for guest vSVA,
> > > device TLB invalidation is passed down. Perhaps we can avoid
> > > passing down dev TLB flush just like what we are doing for guest
> > > IOVA.
> >
> > I think dev TLB is fully handled within IOMMU driver today. It doesn't
> > require device driver to explicit toggle. With this then we can fully
> > virtualize guest dev TLB invalidation request to save one syscall,
> > since the host is supposed to flush dev TLB when serving the earlier
> > IOTLB invalidation pass-down.
> 
> In the previous discussions, we thought about making IOTLB flush
> inclusive, where IOTLB flush would always include device TLB flush. But
> we thought such behavior cannot be assumed for all VMMs, some may still
> do explicit dev TLB flush. So for completeness, we included dev TLB
> here.

is there such example or a link to previous discussion? Here we are
talking about host IOMMU driver behavior, instead of VMM. But I'm
not strong on this, since it's more an optimization. But there remains
one unclear area. If we do want to support such usage with explicit
dev TLB flush, how does host IOMMU driver avoid doing implicit
dev TLB flush when serving iotlb invalidation request? Is it already
designed such way that user-passed-down iotlb invalidation request
only invalidates iotlb while kernel-triggered iotlb invalidation still
does implicit dev TLB flush?

Thanks
Kevin