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

[Jason Wang]

On Wed, Nov 23, 2022 at 1:26 PM Jason Wang <jasowang@redhat.com> wrote:
>
> On Wed, Nov 23, 2022 at 12:28 AM Eric Auger <eric.auger@redhat.com> wrote:
> >
> > Hi,
> >
> > On 11/22/22 10:43, Michael S. Tsirkin wrote:
> > > On Tue, Nov 22, 2022 at 11:01:11AM +0800, 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.
> > > Noted how exactly? What does "using 1G memory" mean?
> >
> > We hit the following assertion:
> >
> > qemu-system-x86_64: ../hw/virtio/vhost.c:85: vhost_dev_sync_region: 
> > Assertion `end / VHOST_LOG_CHUNK < dev->log_size' failed.
> >
> > On the last time vhost_get_log_size() is called it takes into account 2 
> > regions when computing the log_size:
> > qemu-system-x86_64: vhost_get_log_size region 0 last=0x9ffff updated 
> > log_size=0x3
> > qemu-system-x86_64: vhost_get_log_size region 1 last=0x3fffffff updated 
> > log_size=0x1000
> > so in vhost_migration_log() vhost_get_log_size(dev) returns 0x1000
> >
> > In the test case, memory_region_sync_dirty_bitmap() gets called for 
> > mem-machine_mem, vga.vram (several times) and eventually on pc.bios. This 
> > latter is reponsible for the assertion:
> >
> > qemu-system-x86_64: vhost_log_sync calls sync_dirty_map on pc.bios for the 
> > full range
> > qemu-system-x86_64: vhost_sync_dirty_bitmap calls vhost_dev_sync_region on 
> > region 0
> > qemu-system-x86_64: vhost_dev_sync_region end=0x9ffff < start=0xfffc0000
> > qemu-system-x86_64: vhost_sync_dirty_bitmap calls vhost_dev_sync_region on 
> > region 1
> > qemu-system-x86_64: vhost_dev_sync_region end=0x3fffffff < start=0xfffc0000
> > qemu-system-x86_64: vhost_sync_dirty_bitmap calls vhost_dev_sync_region on 
> > vq 0 <-----
> > qemu-system-x86_64: vhost_dev_sync_region pc.bios mfirst=0xfffc0000 
> > mlast=0xffffffff rfirst=0xfffff240 rlast=0xfffffa45
> > qemu-system-x86_64: vhost_dev_sync_region pc.bios end=0xfffffa45 
> > VHOST_LOG_CHUNK=0x40000 end/VHOST_LOG_CHUNK=0x3fff dev->log_size=0x1000
> > qemu-system-x86_64: ../hw/virtio/vhost.c:85: vhost_dev_sync_region: 
> > Assertion `end / VHOST_LOG_CHUNK < dev->log_size' failed.
> >
> >
> >
> > "using 1G memory": We hit the issue with a guest started with 1GB initial 
> > RAM.
>
> Yes, so in the case the guest iova allocator may try to use an IOVA
> that is beyond 1G.
>
> >
> >
> > >
> > >> Fixes: c471ad0e9bd46 ("vhost_net: device IOTLB support")
> > >> Cc: qemu-stable@nongnu.org
> > >> Reported-by: Yalan Zhang <yalzhang@redhat.com>
> > >> Tested-by: Eric Auger <eric.auger@redhat.com>
> > >> Tested-by: Lei Yang <leiyang@redhat.com>
> > >> Signed-off-by: Jason Wang <jasowang@redhat.com>
> > >> ---
> > >>  hw/virtio/vhost.c | 65 ++++++++++++++++++++++++++++++++---------------
> > >>  1 file changed, 45 insertions(+), 20 deletions(-)
> > >>
> > >> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > >> index d1c4c20b8c..26b319f34e 100644
> > >> --- a/hw/virtio/vhost.c
> > >> +++ b/hw/virtio/vhost.c
> > >> @@ -106,11 +106,30 @@ 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,
> > >>                                     hwaddr last)
> > >>  {
> > >> +    IOMMUTLBEntry iotlb;
> > > why don't we move this inside the scope where it's used?
>
> That's fine.
>
> > >
> > >>      int i;
> > >>      hwaddr start_addr;
> > >>      hwaddr end_addr;
> > >> @@ -132,13 +151,37 @@ static int vhost_sync_dirty_bitmap(struct 
> > >> vhost_dev *dev,
> > >>      }
> > >>      for (i = 0; i < dev->nvqs; ++i) {
> > >>          struct vhost_virtqueue *vq = dev->vqs + i;
> > >> +        hwaddr used_phys = vq->used_phys, used_size = vq->used_size;
> > >> +        hwaddr phys, s;
> > > these two, too.
>
> Right.
>
> > >
> > >>
> > >>          if (!vq->used_phys && !vq->used_size) {
> > >>              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)) {
> > >> +            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 == NULL) {
> > >> +                    return -EINVAL;
> > > I am not sure how this can trigger. I don't like == NULL:
> > > !iotlb.target_as is more succint. But a bigger question is how to
> > > handle this. callers ignore the return value so maybe
> > > log guest error? iommu seems misconfigured ...
>
> Ok.
>
> > >
> > >
> > >> +                }
> > >> +
> > >> +                phys = iotlb.translated_addr;
> > >> +                s = MIN(iotlb.addr_mask + 1, used_size);
> > > Note, that iotlb.translated_addr here is an aligned address and
> > > iotlb.addr_mask + 1 is size from there.
> > >
> > > So I think phys that you want is actually
> > >       phys = iotlb.translated_addr + (used_phys & iotlb.addr_mask);
> > >
> > >
> > >
> > > accordingly, the size would be from there until end of mask:
> > >       s = MIN(iotlb.addr_mask + 1 - (used_phys & iotlb.addr_mask), 
> > > used_size);
> > >
> > >
> > > Also, it bothers me that the math here will give you 0 if addr_mask is
> > > all ones.

So even if addr_mask is all ones, we end up with s = MIN(1, used_size)
which should be fine. The used_size has been validated before to be
non-zero.

Thanks

> Then MIN will give 0 too and we loop forever.  I think this
> > > can not trigger, but I'd rather we play it safe and add outside of MIN
> > > after it's capped to a reasonable value. So we end up with:
> > >
> > >       /* Distance from start of used ring until last byte of IOMMU page */
> > >       s = iotlb.addr_mask - (used_phys & iotlb.addr_mask);
> > >       /* size of used ring, or of the part of it until end of IOMMU page 
> > > */
> > >       s = MIN(s, used_size - 1) + 1;
> > >
> > >
>
> Right.
>
> > >
> > >
> > >
> > >> +
> > >> +                vhost_dev_sync_region(dev, section, start_addr, 
> > >> end_addr, phys,
> > >> +                                      range_get_last(phys, used_size));
> > > why are you syncing used_size here? Shouldn't it be s?
>
> Let me fix this.
>
> Thanks
>
> > >
> > >
> > >
> > >> +                used_size -= s;
> > >> +                used_phys += s;
> > >> +            }
> > >> +        } else {
> > >> +            vhost_dev_sync_region(dev, section, start_addr, end_addr, 
> > >> used_phys,
> > >> +                                  range_get_last(used_phys, used_size));
> > >> +        }
> > >>      }
> > >>      return 0;
> > >>  }
> > >> @@ -306,24 +349,6 @@ 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)
> > >>  {
> > >> --
> > >> 2.25.1
> >
> > Thanks
> >
> > Eric
> >