linux-kernel - RE: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET

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Message-ID: <BN9PR11MB5433E2A78648049A41C484B78C869@BN9PR11MB5433.namprd11.prod.outlook.com>
Date:   Thu, 28 Oct 2021 02:07:46 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Alex Williamson <alex.williamson@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "lkml@...ux.net" <lkml@...ux.net>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "lushenming@...wei.com" <lushenming@...wei.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "david@...son.dropbear.id.au" <david@...son.dropbear.id.au>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>
Subject: RE: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Tuesday, October 26, 2021 7:35 AM
> 
> On Fri, Oct 22, 2021 at 03:08:06AM +0000, Tian, Kevin wrote:
> 
> > > I have no idea what security model makes sense for wbinvd, that is the
> > > major question you have to answer.
> >
> > wbinvd flushes the entire cache in local cpu. It's more a performance
> > isolation problem but nothing can prevent it once the user is allowed
> > to call this ioctl. This is the main reason why wbinvd is a privileged
> > instruction and is emulated by kvm as a nop unless an assigned device
> > has no-snoop requirement. alternatively the user may call clflush
> > which is unprivileged and can invalidate a specific cache line, though
> > not efficient for flushing a big buffer.
> >
> > One tricky thing is that the process might be scheduled to different
> > cpus between writing buffers and calling wbinvd ioctl. Since wbvind
> > only has local behavior, it requires the ioctl to call wbinvd on all
> > cpus that this process has previously been scheduled on.
> 
> That is such a hassle, you may want to re-open this with the kvm
> people as it seems ARM also has different behavior between VM and
> process here.
> 
> The ideal is already not being met, so maybe we can keep special
> casing cache ops?
> 

Now Paolo confirmed wbinvd ioctl is just a thought experiment. 

Then Jason, want to have a clarification on 'keep special casing' here.

Did you mean adopting the vfio model which neither allows the user
to decide no-snoop format nor provides a wbinvd ioctl for the user
to manage buffers used for no-snoop traffic, or still wanting the user 
to decide no-snoop format but not implementing a wbinvd ioctl?

The latter option sounds a bit incomplete from uAPI p.o.v. but it
allows us to stay with one-format-one-ioas policy. And anyway the
userspace can still call clflush to do cacheline-based invalidation,
if necessary.

The former option would force us to support multi-formats-one-ioas.

either case it's iommufd which decides and tells kvm whether wbinvd 
is allowed for a process. 

Thanks
Kevin