Re: [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is protected

[Halil Pasic]

On Fri, 21 Feb 2020 13:59:15 +1100
David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> wrote:

> On Thu, Feb 20, 2020 at 05:13:09PM +0100, Christoph Hellwig wrote:
> > On Thu, Feb 20, 2020 at 05:06:06PM +0100, Halil Pasic wrote:
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > index 867c7ebd3f10..fafc8f924955 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -243,6 +243,9 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
> > >  	if (!virtio_has_iommu_quirk(vdev))
> > >  		return true;
> > >  
> > > +	if (force_dma_unencrypted(&vdev->dev))
> > > +		return true;
> > 
> > Hell no.  This is a detail of the platform DMA direct implementation.
> > Drivers have no business looking at this flag, and virtio finally needs
> > to be fixed to use the DMA API properly for everything but legacy devices.
> 
> So, this patch definitely isn't right as it stands, but I'm struggling
> to understand what it is you're saying is the right way.
> 
> By "legacy devices" I assume you mean pre-virtio-1.0 devices, that
> lack the F_VERSION_1 feature flag.  Is that right?  Because I don't
> see how being a legacy device or not relates to use of the DMA API.
> 
> I *think* what you are suggesting here is that virtio devices that
> have !F_IOMMU_PLATFORM should have their dma_ops set up so that the
> DMA API treats IOVA==PA, which will satisfy what the device expects.
> Then the virtio driver can use the DMA API the same way for both
> F_IOMMU_PLATFORM and !F_IOMMU_PLATFORM devices.

I've considered this idea, and as a matter a fact would be my preferred
solution. It would be, if we could use GFP_DMA when allocating coherent
memory through the DMA API. For CCW devices we *must* have some of the
device accessible memory 31 bit addressable (in physical address space),
because the s390 architecture *mandates it*. I had patches
(https://lkml.org/lkml/2019/9/23/313) that do that but Christoph was not
open to the idea.

Always pushing all the stuff allocated by virtio devices into the DMA
zone is not a good option for us. We did that for protected guests,
because that was what Christoph was willing to accept, but not happy
with it.

I'm willing to go down that rute, but I really need the capability to
tell the DMA API 'do this allocation from ZONE_DMA'. I don't care how.


> 
> But if that works for !F_IOMMU_PLATFORM_DEVICES+F_VERSION_1 devices,
> then AFAICT it will work equally well for legacy devices.

I agree.

> 
> Using the DMA API for *everything* in virtio, legacy or not, seems
> like a reasonable approach to me.  But, AFAICT, that does require the
> DMA layer to have some kind of explicit call to turn on this
> behaviour, which the virtio driver would call during initializsation.
> I don't think we can do it 100% within the DMA layer, because only the
> driver can reasonably know when a device has this weird non-standard
> DMA behaviour.
> 

IMHO one could make the DMA API work with different DMA ops. The
virtio transport or the virtio core can probably intervene. But then, we
(virtio-ccw) would need the iommu=off DMA ops to do the bounce buffering
iff force_dma_unencrypted().

Regards,
Halil

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