linux-kernel - Re: [RFC PATCH 2/2] vfio: Include no-iommu mode

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Message-ID: <20151011091954.GA27451@redhat.com>
Date:	Sun, 11 Oct 2015 12:19:54 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Avi Kivity <avi@...lladb.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>,
	avi@...udius-systems.com, gleb@...lladb.com, corbet@....net,
	bruce.richardson@...el.com, linux-kernel@...r.kernel.org,
	alexander.duyck@...il.com, gleb@...udius-systems.com,
	stephen@...workplumber.org, vladz@...udius-systems.com,
	iommu@...ts.linux-foundation.org, hjk@...sjkoch.de,
	gregkh@...uxfoundation.org
Subject: Re: [RFC PATCH 2/2] vfio: Include no-iommu mode

On Sun, Oct 11, 2015 at 12:03:17PM +0300, Avi Kivity wrote:
> 
> 
> On 10/11/2015 11:57 AM, Michael S. Tsirkin wrote:
> >On Sun, Oct 11, 2015 at 11:12:14AM +0300, Avi Kivity wrote:
> >>>   Mixing no-iommu and secure VFIO is
> >>>also unsupported, as are any VFIO IOMMU backends other than the
> >>>vfio-noiommu backend.  Furthermore, unsafe group files are relocated
> >>>to /dev/vfio-noiommu/.  Upon successful loading in this mode, the
> >>>kernel is tainted due to the dummy IOMMU put in place.  Unloading of
> >>>the module in this mode is also unsupported and will BUG due to the
> >>>lack of support for unregistering an IOMMU for a bus type.
> >>I did not see an API for detecting whether memory translation is provided or
> >>not.  We can have the caller guess this by looking at the device name, or by
> >>requiring the user to specify this, but I think it's cleaner to provide
> >>programmatic access to this attribute.
> >It seems that caller can just check for VFIO_NOIOMMU_IOMMU.
> >
> >Isn't this why it's there?
> 
> That's just means the capability is there, not that it's active.

Well it's currently exactly the same.
I guess you can check the return value of VFIO_SET_IOMMU as well.

> But since you must pass the same value to open(), you already know that
> you're using noiommu.
> 
> >VFIO_IOMMU_MAP_DMA, VFIO_IOMMU_ENABLE and VFIO_IOMMU_DISABLE
> >will probably also fail ...
> >
> 
> Don't you have to call MAP_DMA to pin the memory?

Well check it out - the patch in question doesn't implement this ioctl.
In fact it doesn't implement anything except CHECK_EXTENSION.

And this makes sense to me: MAP_DMA
maps a virtual address to io address, and that doesn't
work for the dummy iommu.

You can pin memory using many other ways, including
mlock and hugetlbfs.

-- 
MST
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