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

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

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?

Also, if it's PT, can't we bypass iommu altogether? That would be
even faster ...

> ---
>  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.
> +        */
> +
> +        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