Re: [Qemu-devel] [PATCH v4 10/10] vhost: iommu: cache static mapping if there is

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, May 22, 2017 at 10:42:00AM +0800, Peter Xu wrote:
> On Fri, May 19, 2017 at 07:55:26PM +0300, Michael S. Tsirkin wrote:
> > On Fri, May 19, 2017 at 11:19:49AM +0800, Peter Xu wrote:
> > > This patch pre-heat vhost iotlb cache when passthrough mode enabled.
> > > 
> > > Sometimes, even if user specified iommu_platform for vhost devices,
> > > IOMMU might still be disabled. One case is passthrough mode in VT-d
> > > implementation. We can detect this by observing iommu_list. If it's
> > > empty, it means IOMMU translation is disabled, then we can actually
> > > pre-heat the translation (it'll be static mapping then) by first
> > > invalidating all IOTLB, then cache existing memory ranges into vhost
> > > backend iotlb using 1:1 mapping.
> > > 
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > 
> > I don't really understand. Is this a performance optimization?
> > Can you post some #s please?
> 
> Yes, it is. Vhost can work even without this patch, but it should be
> faster when with this patch.

You'll have to include perf testing numbers then.

> As mentioned in the commit message and below comment [1], this patch
> pre-heat the cache for vhost. Currently the cache entries depends on
> the system memory ranges (dev->mem->nregions), and it should be far
> smaller than vhost's cache count (currently it is statically defined
> as max_iotlb_entries=2048 in kernel). If with current patch, these
> cache entries can cover the whole possible DMA ranges that PT mode
> would allow, so we won't have any cache miss then.
> 
> For the comments, do you have any better suggestion besides commit
> message and [1]?
> 
> > 
> > Also, if it's PT, can't we bypass iommu altogether? That would be
> > even faster ...
> 
> Yes, but I don't yet know a good way to do it... Any suggestion is
> welcomed as well.
> 
> Btw, do you have any comment on other patches besides this one? Since
> this patch can really be isolated from the whole PT support series.
> 
> Thanks,

I've applied the rest of the series.

> > 
> > > ---
> > >  hw/virtio/trace-events |  4 ++++
> > >  hw/virtio/vhost.c      | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
> > >  2 files changed, 53 insertions(+)
> > > 
> > > diff --git a/hw/virtio/trace-events b/hw/virtio/trace-events
> > > index 1f7a7c1..54dcbb3 100644
> > > --- a/hw/virtio/trace-events
> > > +++ b/hw/virtio/trace-events
> > > @@ -24,3 +24,7 @@ virtio_balloon_handle_output(const char *name, uint64_t gpa) "section name: %s g
> > >  virtio_balloon_get_config(uint32_t num_pages, uint32_t actual) "num_pages: %d actual: %d"
> > >  virtio_balloon_set_config(uint32_t actual, uint32_t oldactual) "actual: %d oldactual: %d"
> > >  virtio_balloon_to_target(uint64_t target, uint32_t num_pages) "balloon target: %"PRIx64" num_pages: %d"
> > > +
> > > +# hw/virtio/vhost.c
> > > +vhost_iommu_commit(void) ""
> > > +vhost_iommu_static_preheat(void) ""
> > > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > > index 03a46a7..8069135 100644
> > > --- a/hw/virtio/vhost.c
> > > +++ b/hw/virtio/vhost.c
> > > @@ -27,6 +27,7 @@
> > >  #include "hw/virtio/virtio-access.h"
> > >  #include "migration/blocker.h"
> > >  #include "sysemu/dma.h"
> > > +#include "trace.h"
> > >  
> > >  /* enabled until disconnected backend stabilizes */
> > >  #define _VHOST_DEBUG 1
> > > @@ -730,6 +731,11 @@ static void vhost_iommu_unmap_notify(IOMMUNotifier *n, IOMMUTLBEntry *iotlb)
> > >      }
> > >  }
> > >  
> > > +static bool vhost_iommu_mr_enabled(struct vhost_dev *dev)
> > > +{
> > > +    return !QLIST_EMPTY(&dev->iommu_list);
> > > +}
> > > +
> > >  static void vhost_iommu_region_add(MemoryListener *listener,
> > >                                     MemoryRegionSection *section)
> > >  {
> > > @@ -782,6 +788,48 @@ static void vhost_iommu_region_del(MemoryListener *listener,
> > >      }
> > >  }
> > >  
> > > +static void vhost_iommu_commit(MemoryListener *listener)
> > > +{
> > > +    struct vhost_dev *dev = container_of(listener, struct vhost_dev,
> > > +                                         iommu_listener);
> > > +    struct vhost_memory_region *r;
> > > +    int i;
> > > +
> > > +    trace_vhost_iommu_commit();
> > > +
> > > +    if (!vhost_iommu_mr_enabled(dev)) {
> > > +        /*
> > > +        * This means iommu_platform is enabled, however iommu memory
> > > +        * region is disabled, e.g., when device passthrough is setup.
> > > +        * Then, no translation is needed any more.
> > > +        *
> > > +        * Let's first invalidate the whole IOTLB, then pre-heat the
> > > +        * static mapping by looping over vhost memory ranges.
> > > +        */
> 
> [1]
> 
> > > +
> > > +        if (dev->vhost_ops->vhost_invalidate_device_iotlb(dev, 0,
> > > +                                                          UINT64_MAX)) {
> > > +            error_report("%s: flush existing IOTLB failed", __func__);
> > > +            return;
> > > +        }
> > > +
> > > +        for (i = 0; i < dev->mem->nregions; i++) {
> > > +            r = &dev->mem->regions[i];
> > > +            /* Vhost regions are writable RAM, so IOMMU_RW suites. */
> > > +            if (dev->vhost_ops->vhost_update_device_iotlb(dev,
> > > +                                                          r->guest_phys_addr,
> > > +                                                          r->userspace_addr,
> > > +                                                          r->memory_size,
> > > +                                                          IOMMU_RW)) {
> > > +                error_report("%s: pre-heat static mapping failed", __func__);
> > > +                return;
> > > +            }
> > > +        }
> > > +
> > > +        trace_vhost_iommu_static_preheat();
> > > +    }
> > > +}
> > > +
> > >  static void vhost_region_nop(MemoryListener *listener,
> > >                               MemoryRegionSection *section)
> > >  {
> > > @@ -1298,6 +1346,7 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
> > >      hdev->iommu_listener = (MemoryListener) {
> > >          .region_add = vhost_iommu_region_add,
> > >          .region_del = vhost_iommu_region_del,
> > > +        .commit = vhost_iommu_commit,
> > >      };
> > >  
> > >      if (hdev->migration_blocker == NULL) {
> > > -- 
> > > 2.7.4
> 
> -- 
> Peter Xu