Linux Virtualization - Sep 2014 - [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

On Wed, 2014-09-03 at 09:47 +0200, Paolo Bonzini wrote:
> 
> IOMMU support for x86 is going to go in this week.
But won't that break virtio on x86 ? Or will virtio continue bypassing
it ? IE, the guest side virtio doesn't expect an IOMMU and doesn't call
the dma mappings ops.
> However, it is and likely will remain niche enough that I don't really
> care about performance loss from IOMMU support.  If you enable it, you
> want it.
> 
> So from the QEMU point of view we can simply add the direct-ram-access
> property, and have the pseries machine turn it on by default (while
> other machines can leave it off by default---they have no IOMMU and
> thus no performance cost).
Well, it's only for virtio and should be on by default on x86 as well if
an iommu is installed no ?

Cheers,
Ben.

Il 03/09/2014 10:05, Benjamin Herrenschmidt ha scritto:
> On Wed, 2014-09-03 at 09:47 +0200, Paolo Bonzini wrote:
>>
>> IOMMU support for x86 is going to go in this week.
> 
> But won't that break virtio on x86 ? Or will virtio continue bypassing
> it ? IE, the guest side virtio doesn't expect an IOMMU and doesn't
call
> the dma mappings ops.
> 
>> However, it is and likely will remain niche enough that I don't
really
>> care about performance loss from IOMMU support.  If you enable it, you
>> want it.
>>
>> So from the QEMU point of view we can simply add the direct-ram-access
>> property, and have the pseries machine turn it on by default (while
>> other machines can leave it off by default---they have no IOMMU and
>> thus no performance cost).
> 
> Well, it's only for virtio and should be on by default on x86 as well
if
> an iommu is installed no ?
Yes, only for virtio---but for x86 I think it should be off by default,
even if that means virtio+IOMMU requires a new kernel.

Paolo

On Sep 3, 2014 5:11 AM, "Paolo Bonzini" <pbonzini at redhat.com>
wrote:
>
> Il 03/09/2014 10:05, Benjamin Herrenschmidt ha scritto:
> > On Wed, 2014-09-03 at 09:47 +0200, Paolo Bonzini wrote:
> >>
> >> IOMMU support for x86 is going to go in this week.
> >
> > But won't that break virtio on x86 ? Or will virtio continue
bypassing
> > it ? IE, the guest side virtio doesn't expect an IOMMU and
doesn't call
> > the dma mappings ops.
> >
> >> However, it is and likely will remain niche enough that I
don't really
> >> care about performance loss from IOMMU support.  If you enable it,
you
> >> want it.
> >>
> >> So from the QEMU point of view we can simply add the
direct-ram-access
> >> property, and have the pseries machine turn it on by default
(while
> >> other machines can leave it off by default---they have no IOMMU
and
> >> thus no performance cost).
> >
> > Well, it's only for virtio and should be on by default on x86 as
well if
> > an iommu is installed no ?
>
> Yes, only for virtio---but for x86 I think it should be off by default,
> even if that means virtio+IOMMU requires a new kernel.
Just to clarify: is "it" the direct-ram-access property?  If so, I
think I might agree.

Alternatively, could QEMU easily teach the IOMMU code to generate the
ACPI tables such that virtio-pci devices aren't advertised as living
behind the IOMMU?  This would work both with and without my patches.
On the other hand, maybe this gets complicated when hotplug is
involved.

--Andy
>
> Paolo