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

[Jason Wang]

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. 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
>