Linux Virtualization - Apr 2016 - [PATCH V2 RFC] fixup! virtio: convert to use DMA api

On Wed, Apr 27, 2016 at 04:37:04PM +0300, Michael S. Tsirkin
wrote:
> One correction: it's a feature of the device in the system.
> There could be a mix of devices bypassing and not
> bypassing the IOMMU.
No, it really is not. A device can't chose to bypass the IOMMU. But the
IOMMU can chose to let the device bypass. So any fix here belongs
into the platform/iommu code too and not into some driver.
> Sounds good. And a way to detect appropriate devices could
> be by looking at the feature flag, perhaps?
Again, no! The way to detect that is to look into the iommu description
structures provided by the firmware. They provide everything necessary
to tell the iommu code which devices are not translated.



	Joerg

On Wed, Apr 27, 2016 at 7:23 AM, Joerg Roedel <joro at 8bytes.org>
wrote:
> On Wed, Apr 27, 2016 at 04:37:04PM +0300, Michael S. Tsirkin wrote:
>> One correction: it's a feature of the device in the system.
>> There could be a mix of devices bypassing and not
>> bypassing the IOMMU.
>
> No, it really is not. A device can't chose to bypass the IOMMU. But the
> IOMMU can chose to let the device bypass. So any fix here belongs
> into the platform/iommu code too and not into some driver.
>
>> Sounds good. And a way to detect appropriate devices could
>> be by looking at the feature flag, perhaps?
>
> Again, no! The way to detect that is to look into the iommu description
> structures provided by the firmware. They provide everything necessary
> to tell the iommu code which devices are not translated.
>
Except on PPC and SPARC.  As far as I know, those are the only
problematic platforms.

Is it too late to *disable* QEMU's q35-iommu thingy until it can be
fixed to report correct data in the DMAR tables?

--Andy

On Wed, Apr 27, 2016 at 04:23:32PM +0200, Joerg Roedel
wrote:
> On Wed, Apr 27, 2016 at 04:37:04PM +0300, Michael S. Tsirkin wrote:
> > One correction: it's a feature of the device in the system.
> > There could be a mix of devices bypassing and not
> > bypassing the IOMMU.
> 
> No, it really is not. A device can't chose to bypass the IOMMU. But the
> IOMMU can chose to let the device bypass.
QEMU can choose to bypass IOMMU for one device and not the other.
IOMMU in QEMU isn't involved when it's bypassed.
> So any fix here belongs
> into the platform/iommu code too and not into some driver.
Fine but this is beside the point. Almost all virtio devices
bypass IOMMU and what this patch does is create a way
to detect devices that don't. This code can maybe go into
platform.
> > Sounds good. And a way to detect appropriate devices could
> > be by looking at the feature flag, perhaps?
> 
> Again, no! The way to detect that is to look into the iommu description
> structures provided by the firmware. They provide everything necessary
> to tell the iommu code which devices are not translated.
> 
> 
> 
> 	Joerg
It would be easy if they did but they don't do this on all systems.
In particular the idea for firmware interface was clearly
that a given bus either is connected through IOMMU or bypassing it.
Whether virtio bypasses the iommu is unrelated to the bus it's on.

-- 
MST

On Wed, Apr 27, 2016 at 07:31:43AM -0700, Andy Lutomirski
wrote:
> On Wed, Apr 27, 2016 at 7:23 AM, Joerg Roedel <joro at 8bytes.org>
wrote:
> > On Wed, Apr 27, 2016 at 04:37:04PM +0300, Michael S. Tsirkin wrote:
> >> One correction: it's a feature of the device in the system.
> >> There could be a mix of devices bypassing and not
> >> bypassing the IOMMU.
> >
> > No, it really is not. A device can't chose to bypass the IOMMU.
But the
> > IOMMU can chose to let the device bypass. So any fix here belongs
> > into the platform/iommu code too and not into some driver.
> >
> >> Sounds good. And a way to detect appropriate devices could
> >> be by looking at the feature flag, perhaps?
> >
> > Again, no! The way to detect that is to look into the iommu
description
> > structures provided by the firmware. They provide everything necessary
> > to tell the iommu code which devices are not translated.
> >
> 
> Except on PPC and SPARC.  As far as I know, those are the only
> problematic platforms.
> 
> Is it too late to *disable* QEMU's q35-iommu thingy until it can be
> fixed to report correct data in the DMAR tables?
> 
> --Andy
Meaning virtio or assigned devices?
For virtio - it's way too late since these are working configurations.
For assigned devices - they don't work on x86 so it doesn't have
to be disabled, it's safe to ignore.

-- 
MST

On Wed, Apr 27, 2016 at 05:34:30PM +0300, Michael S. Tsirkin
wrote:
> On Wed, Apr 27, 2016 at 04:23:32PM +0200, Joerg Roedel wrote:
> QEMU can choose to bypass IOMMU for one device and not the other.
> IOMMU in QEMU isn't involved when it's bypassed.
And it is QEMU's task to tell the OS, right? And the correct way to do
this is via the firmware ACPI tables.
> Fine but this is beside the point. Almost all virtio devices
> bypass IOMMU and what this patch does is create a way
> to detect devices that don't. This code can maybe go into
> platform.
Again, the way to detect this is in platform code must not be device
specific. This is what the DMAR and IVRS tables on x86 are for.

When there is no way to do this in the firmware (or there is no firmware
at all), we have to do a quirk in the platform code for it.



	Joerg