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Date:   Fri, 4 Jun 2021 09:37:55 -0600
From:   Alex Williamson <alex.williamson@redhat.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
Cc:     Jason Gunthorpe <jgg@nvidia.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Robin Murphy <robin.murphy@arm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        David Gibson <david@gibson.dropbear.id.au>,
        Kirti Wankhede <kwankhede@nvidia.com>,
        "David Woodhouse" <dwmw2@infradead.org>,
        Jason Wang <jasowang@redhat.com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal
Message-ID: <20210604093755.1d660a47.alex.williamson@redhat.com>
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On Fri, 4 Jun 2021 09:19:50 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Alex Williamson <alex.williamson@redhat.com>
> > Sent: Friday, June 4, 2021 4:42 AM
> >   
> > > 'qemu --allow-no-snoop' makes more sense to me  
> > 
> > I'd be tempted to attach it to the -device vfio-pci option, it's
> > specific drivers for specific devices that are going to want this and
> > those devices may not be permanently attached to the VM.  But I see in
> > the other thread you're trying to optimize IOMMU page table sharing.
> > 
> > There's a usability question in either case though and I'm not sure how
> > to get around it other than QEMU or the kernel knowing a list of
> > devices (explicit IDs or vendor+class) to select per device defaults.
> >   
> 
> "-device vfio-pci" is a per-device option, which implies that the
> no-snoop choice is given to the admin then no need to maintain 
> a fixed device list in Qemu?

I think we want to look at where we put it to have the best default
user experience.  For example the QEMU vfio-pci device option could use
on/off/auto semantics where auto is the default and QEMU maintains a
list of IDs or vendor/class configurations where we've determined the
"optimal" auto configuration.  Management tools could provide an
override, but we're imposing some pretty technical requirements for a
management tool to be able to come up with good per device defaults.
Seems like we should consolidate that technical decision in one place.
Thanks,

Alex