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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 25 May 2018 20:45:42 +0300

On Thu, May 24, 2018 at 08:27:04AM +1000, Benjamin Herrenschmidt wrote:
> 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 ?

Not sure I understand. Just set the flag e.g. on qemu command line.
I might be wrong, but these secure mode things usually
a. require hypervisor side tricks anyway

> - 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.

Well it's not supposed to be much slower for the static case.

vhost has a cache so should be fine.

A while ago Paolo implemented a translation cache which should be
perfect for this case - most of the code got merged but
never enabled because of stability issues.

If all else fails, we could teach QEMU to handle the no-iommu case
as if VIRTIO_F_IOMMU_PLATFORM was off.

> > 
> > 
> > >  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