linux-kernel - Re: [RFC] /dev/ioasid uAPI proposal

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Date:   Fri, 4 Jun 2021 15:45:11 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Robin Murphy <robin.murphy@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Kirti Wankhede <kwankhede@...dia.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On Fri, 4 Jun 2021 09:13:37 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Thu, Jun 03, 2021 at 02:41:36PM -0600, Alex Williamson wrote:
> 
> > Could you clarify "vfio_driver"?    
> 
> This is the thing providing the vfio_device_ops function pointers.
> 
> So vfio-pci can't know anything about this (although your no-snoop
> control probing idea makes sense to me)
> 
> But vfio_mlx5_pci can know
> 
> So can mdev_idxd
> 
> And kvmgt

A capability on VFIO_DEVICE_GET_INFO could provide a hint to userspace.
Stock vfio-pci could fill it out to the extent advertising if the
device is capable of non-coherent DMA based on the Enable No-snoop
probing, the device specific vfio_drivers could set it based on
knowledge of the device behavior.  Another bit might indicate a
preference to not suppress non-coherent DMA at the IOMMU.  Thanks,

Alex