Re: [RFC V2] virtio: Add platform specific DMA API translation for virito devices

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2018-05-23 at 21:50 +0300, Michael S. Tsirkin wrote:

> I re-read that discussion and I'm still unclear on the
> original question, since I got several apparently
> conflicting answers.
> 
> I asked:
> 
> 	Why isn't setting VIRTIO_F_IOMMU_PLATFORM on the
> 	hypervisor side sufficient?

I thought I had replied to this...

There are a couple of reasons:

- First qemu doesn't know that the guest will switch to "secure mode"
in advance. There is no difference between a normal and a secure
partition until the partition does the magic UV call to "enter secure
mode" and qemu doesn't see any of it. So who can set the flag here ?

- Second, when using VIRTIO_F_IOMMU_PLATFORM, we also make qemu (or
vhost) go through the emulated MMIO for every access to the guest,
which adds additional overhead.

Cheers,
Ben.

> 
> 
> >  arch/powerpc/include/asm/dma-mapping.h |  6 ++++++
> >  arch/powerpc/platforms/pseries/iommu.c | 11 +++++++++++
> >  drivers/virtio/virtio_ring.c           | 10 ++++++++++
> >  3 files changed, 27 insertions(+)
> > 
> > diff --git a/arch/powerpc/include/asm/dma-mapping.h b/arch/powerpc/include/asm/dma-mapping.h
> > index 8fa3945..056e578 100644
> > --- a/arch/powerpc/include/asm/dma-mapping.h
> > +++ b/arch/powerpc/include/asm/dma-mapping.h
> > @@ -115,4 +115,10 @@ extern u64 __dma_get_required_mask(struct device *dev);
> >  #define ARCH_HAS_DMA_MMAP_COHERENT
> >  
> >  #endif /* __KERNEL__ */
> > +
> > +#define platform_forces_virtio_dma platform_forces_virtio_dma
> > +
> > +struct virtio_device;
> > +
> > +extern bool platform_forces_virtio_dma(struct virtio_device *vdev);
> >  #endif	/* _ASM_DMA_MAPPING_H */
> > diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
> > index 06f0296..a2ec15a 100644
> > --- a/arch/powerpc/platforms/pseries/iommu.c
> > +++ b/arch/powerpc/platforms/pseries/iommu.c
> > @@ -38,6 +38,7 @@
> >  #include <linux/of.h>
> >  #include <linux/iommu.h>
> >  #include <linux/rculist.h>
> > +#include <linux/virtio.h>
> >  #include <asm/io.h>
> >  #include <asm/prom.h>
> >  #include <asm/rtas.h>
> > @@ -1396,3 +1397,13 @@ static int __init disable_multitce(char *str)
> >  __setup("multitce=", disable_multitce);
> >  
> >  machine_subsys_initcall_sync(pseries, tce_iommu_bus_notifier_init);
> > +
> > +bool platform_forces_virtio_dma(struct virtio_device *vdev)
> > +{
> > +	/*
> > +	 * On protected guest platforms, force virtio core to use DMA
> > +	 * MAP API for all virtio devices. But there can also be some
> > +	 * exceptions for individual devices like virtio balloon.
> > +	 */
> > +	return (of_find_compatible_node(NULL, NULL, "ibm,ultravisor") != NULL);
> > +}
> 
> Isn't this kind of slow?  vring_use_dma_api is on
> data path and supposed to be very fast.
> 
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 21d464a..47ea6c3 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -141,8 +141,18 @@ struct vring_virtqueue {
> >   * unconditionally on data path.
> >   */
> >  
> > +#ifndef platform_forces_virtio_dma
> > +static inline bool platform_forces_virtio_dma(struct virtio_device *vdev)
> > +{
> > +	return false;
> > +}
> > +#endif
> > +
> >  static bool vring_use_dma_api(struct virtio_device *vdev)
> >  {
> > +	if (platform_forces_virtio_dma(vdev))
> > +		return true;
> > +
> >  	if (!virtio_has_iommu_quirk(vdev))
> >  		return true;
> >  
> > -- 
> > 2.9.3