linux-kernel - Re: [PATCH v4 09/12] iommu/vt-d: Add iotlb flush for nested domain

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Message-ID: <ZNUI0D7ZMvLWlBNx@nvidia.com>
Date:   Thu, 10 Aug 2023 12:57:04 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Nicolin Chen <nicolinc@...dia.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>
Subject: Re: [PATCH v4 09/12] iommu/vt-d: Add iotlb flush for nested domain

On Thu, Aug 10, 2023 at 02:49:59AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@...dia.com>
> > Sent: Thursday, August 10, 2023 4:17 AM
> > 
> > On Wed, Aug 09, 2023 at 04:19:01PM -0300, Jason Gunthorpe wrote:
> > > On Wed, Aug 09, 2023 at 12:12:25PM -0700, Nicolin Chen wrote:
> > > > On Wed, Aug 09, 2023 at 01:24:56PM -0300, Jason Gunthorpe wrote:
> > > > > Similarly for managing the array of invalidation commands.
> > > >
> > > > You mean an embedded uptr inside a driver user data struct right?
> > > > Sure, that should go through the new helper too.
> > >
> > > If we are committed that all drivers have to process an array then put
> > > the array in the top level struct and pass it in the same user_data
> > > struct and use another helper to allow the driver to iterate through
> > > it.
> > 
> > I see. Both VTD and SMMU pass uptr to the arrays of invalidation
> > commands/requests. The only difference is that SMMU's array is a
> > ring buffer other than a plain one indexing from the beginning.
> > But the helper could take two index inputs, which should work for
> > VTD case too. If another IOMMU driver only supports one request,
> > rather than a array of requests, we can treat that as a single-
> > entry array.
> > 
> 
> I like this approach.

Do we need to worry about the ring wrap around? It is already the case
that the VMM has to scan the ring and extract the invalidation
commands, wouldn't it already just linearize them?

Is there a use case for invaliation only SW emulated rings, and do we
care about optimizing for the wrap around case?

Let's answer those questions before designing something complicated :)

Jason