Re: [PATCH for 7.2? V2] vhost: fix vq dirty bitmap syncing when vIOMMU is enabled

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

On Tue, Nov 29, 2022 at 10:52:29AM +0100, Eric Auger wrote:
> Hi Jason,
> 
> On 11/29/22 05:02, Jason Wang wrote:
> > When vIOMMU is enabled, the vq->used_phys is actually the IOVA not
> > GPA. So we need to translate it to GPA before the syncing otherwise we
> > may hit the following crash since IOVA could be out of the scope of
> > the GPA log size. This could be noted when using virtio-IOMMU with
> > vhost using 1G memory.
> >
> > Fixes: c471ad0e9bd46 ("vhost_net: device IOTLB support")
> > Cc: qemu-stable@nongnu.org
> > Tested-by: Lei Yang <leiyang@redhat.com>
> > Reported-by: Yalan Zhang <yalzhang@redhat.com>
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > ---
> > Changes since V1:
> > - Fix the address calculation when used ring is not page aligned
> > - Fix the length for each round of dirty bitmap syncing
> > - Use LOG_GUEST_ERROR to log wrong used adddress
> > - Various other tweaks
> > ---
> >  hw/virtio/vhost.c | 76 ++++++++++++++++++++++++++++++++++-------------
> >  1 file changed, 56 insertions(+), 20 deletions(-)
> >
> > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > index d1c4c20b8c..0cd5f25fcb 100644
> > --- a/hw/virtio/vhost.c
> > +++ b/hw/virtio/vhost.c
> > @@ -20,6 +20,7 @@
> >  #include "qemu/range.h"
> >  #include "qemu/error-report.h"
> >  #include "qemu/memfd.h"
> > +#include "qemu/log.h"
> >  #include "standard-headers/linux/vhost_types.h"
> >  #include "hw/virtio/virtio-bus.h"
> >  #include "hw/virtio/virtio-access.h"
> > @@ -106,6 +107,24 @@ static void vhost_dev_sync_region(struct vhost_dev *dev,
> >      }
> >  }
> >  
> > +static bool vhost_dev_has_iommu(struct vhost_dev *dev)
> > +{
> > +    VirtIODevice *vdev = dev->vdev;
> > +
> > +    /*
> > +     * For vhost, VIRTIO_F_IOMMU_PLATFORM means the backend support
> > +     * incremental memory mapping API via IOTLB API. For platform that
> > +     * does not have IOMMU, there's no need to enable this feature
> > +     * which may cause unnecessary IOTLB miss/update transactions.
> > +     */
> > +    if (vdev) {
> > +        return virtio_bus_device_iommu_enabled(vdev) &&
> > +            virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
> > +    } else {
> > +        return false;
> > +    }
> > +}
> > +
> >  static int vhost_sync_dirty_bitmap(struct vhost_dev *dev,
> >                                     MemoryRegionSection *section,
> >                                     hwaddr first,
> > @@ -137,8 +156,43 @@ static int vhost_sync_dirty_bitmap(struct vhost_dev *dev,
> >              continue;
> >          }
> >  
> > -        vhost_dev_sync_region(dev, section, start_addr, end_addr, vq->used_phys,
> > -                              range_get_last(vq->used_phys, vq->used_size));
> > +        if (vhost_dev_has_iommu(dev)) {
> > +            IOMMUTLBEntry iotlb;
> > +            hwaddr used_phys = vq->used_phys, used_size = vq->used_size;
> > +            hwaddr phys, s;
> > +
> > +            while (used_size) {
> > +                rcu_read_lock();
> > +                iotlb = address_space_get_iotlb_entry(dev->vdev->dma_as,
> > +                                                      used_phys,
> > +                                                      true, MEMTXATTRS_UNSPECIFIED);
> > +                rcu_read_unlock();
> > +
> > +                if (!iotlb.target_as) {
> > +                    qemu_log_mask(LOG_GUEST_ERROR, "translation "
> > +                                  "failure for used_phys %"PRIx64"\n", used_phys);
> looks weird to see translation of "used_phys" whereas it is an iova. At
> least I would reword the msg
> > +                    return -EINVAL;
> > +                }
> > +
> > +                phys = iotlb.translated_addr + (used_phys & iotlb.addr_mask);
> you may use a local variable storing this offset =
> 
> used_phys & iotlb.addr_mask
> 
> > +
> > +                /* Distance from start of used ring until last byte of
> > +                   IOMMU page */
> you can avoid checkpatch warnings here
> > +                s = iotlb.addr_mask - (used_phys & iotlb.addr_mask);
> > +                /* Size of used ring, or of the part of it until end
> > +                   of IOMMU page */
> and here
> 
> I would suggest to rewrite this into
> s =iotlb.addr_mask - (used_phys & iotlb.addr_mask) + 1
> s = MIN(s, used_size);

This does not work - if iotlb.addr_mask - (used_phys & iotlb.addr_mask)
is all-ones then + 1 gives you 0 and MIN gives you 0.
Theoretical but worth being safe here IMHO.


> > +                s = MIN(s, used_size - 1) + 1;
> > +
> > +                vhost_dev_sync_region(dev, section, start_addr, end_addr, phys,
> > +                                      range_get_last(phys, s));
> > +                used_size -= s;
> > +                used_phys += s;
> > +            }
> > +        } else {
> > +            vhost_dev_sync_region(dev, section, start_addr,
> > +                                  end_addr, vq->used_phys,
> > +                                  range_get_last(vq->used_phys, vq->used_size));
> > +        }
> >      }
> >      return 0;
> >  }
> > @@ -306,24 +360,6 @@ static inline void vhost_dev_log_resize(struct vhost_dev *dev, uint64_t size)
> >      dev->log_size = size;
> >  }
> >  
> > -static bool vhost_dev_has_iommu(struct vhost_dev *dev)
> > -{
> > -    VirtIODevice *vdev = dev->vdev;
> > -
> > -    /*
> > -     * For vhost, VIRTIO_F_IOMMU_PLATFORM means the backend support
> > -     * incremental memory mapping API via IOTLB API. For platform that
> > -     * does not have IOMMU, there's no need to enable this feature
> > -     * which may cause unnecessary IOTLB miss/update transactions.
> > -     */
> > -    if (vdev) {
> > -        return virtio_bus_device_iommu_enabled(vdev) &&
> > -            virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
> > -    } else {
> > -        return false;
> > -    }
> > -}
> > -
> >  static void *vhost_memory_map(struct vhost_dev *dev, hwaddr addr,
> >                                hwaddr *plen, bool is_write)
> >  {
> Besides,
> 
> Tested-by: Eric Auger <eric.auger@redhat.com>
> 
> Eric