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

[Eric Auger <eric.auger@redhat.com>]

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.


>
>> 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?
>
>>      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.
>
>>  
>>          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 ...
>
>
>> +                }
>> +
>> +                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;
>
>
>
>
>
>> +
>> +                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?
>
>
>
>> +                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