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

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Date:   Fri, 29 Oct 2021 10:55:18 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.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

On Thu, Oct 28, 2021 at 02:07:46AM +0000, Tian, Kevin wrote:
> > 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?

IMHO if the wbinvd is just a thought experiment then userspace should
directly control the wbinvd enable and present the iommufd as 'proof'
to enable it.

The thought experiment tells us that iommufd should have a wbinvd
ioctl, even if we don't implement it today. So access to iommufd is
also access to wbinvd.

iommufd should control/report via intel sepecific areas if the IOS is
no-snoop blocking or not so userspace can decide what it wants to do.

Jason