Re: [PATCH RFC 00/12] vdpa: generalize vdpa simulator and add block device

Jason Wang
2020-12-22T02:45:11Z

On 2020/12/21 下午7:14, Stefano Garzarella wrote:
> On Mon, Dec 21, 2020 at 11:16:54AM +0800, Jason Wang wrote:
>>
>> On 2020/12/18 下午7:38, Stefano Garzarella wrote:
>>> On Mon, Nov 16, 2020 at 11:37:48AM +0800, Jason Wang wrote:
>>>>
>>>> On 2020/11/13 下午9:47, Stefano Garzarella wrote:
>>>>> Thanks to Max that started this work!
>>>>> I took his patches, and extended the block simulator a bit.
>>>>>
>>>>> This series moves the network device simulator in a new module
>>>>> (vdpa_sim_net) and leaves the generic functions in the vdpa_sim core
>>>>> module, allowing the possibility to add new vDPA device simulators.
>>>>> Then we added a new vdpa_sim_blk module to simulate a block device.
>>>>>
>>>>> I'm not sure about patch 11 ("vringh: allow vringh_iov_xfer() to skip
>>>>> bytes when ptr is NULL"), maybe we can add a new functions instead of
>>>>> modify vringh_iov_xfer().
>>>>>
>>>>> As Max reported, I'm also seeing errors with vdpa_sim_blk related to
>>>>> iotlb and vringh when there is high load, these are some of the error
>>>>> messages I can see randomly:
>>>>>
>>>>>   vringh: Failed to access avail idx at 00000000e8deb2cc
>>>>>   vringh: Failed to read head: idx 6289 address 00000000e1ad1d50
>>>>>   vringh: Failed to get flags at 000000006635d7a3
>>>>>
>>>>>   virtio_vdpa vdpa0: vringh_iov_push_iotlb() error: -14 offset:
>>>>> 0x2840000 len: 0x20000
>>>>>   virtio_vdpa vdpa0: vringh_iov_pull_iotlb() error: -14 offset:
>>>>> 0x58ee000 len: 0x3000
>>>>>
>>>>> These errors should all be related to the fact that iotlb_translate()
>>>>> fails with -EINVAL, so it seems that we miss some mapping.
>>>>
>>>>
>>>> Is this only reproducible when there's multiple co-current
>>>> accessing of IOTLB? If yes, it's probably a hint that some kind of
>>>> synchronization is still missed somewhere.
>>>>
>>>> It might be useful to log the dma_map/unmp in both virtio_ring and
>>>> vringh to see who is missing the map.
>>>>
>>>
>>> Just an update about these issues with vdpa-sim-blk.
>>> I've been focusing a little bit on these failures over the last few
>>> days and have found two issues related to the IOTLB/IOMMU:
>>>
>>> 1. Some requests coming from the block layer fills the SG list with
>>> multiple buffers that had the same physical address. This happens
>>> for example while using 'mkfs', at some points multiple sectors are
>>> zeroed so multiple SG elements point to the same physical page that
>>> is zeroed.
>>> Since we are using vhost_iotlb_del_range() in the
>>> vdpasim_unmap_page(), this removes all the overlapped ranges. I
>>> fixed removing a single map in vdpasim_unmap_page(), but has an
>>> alternative we can implement some kind of reference counts.
>>
>>
>> I think we need to do what hardware do. So using refcount is probably
>> not a good ida.
>
> Okay, so since we are using for simplicity an identical mapping, we
> are assigning the same dma_addr to multiple pages.

I think I get you now. That's the root cause for the failure.

Then I think we need an simple iova allocator for vdpa simulator, and it
might be useful for VDUSE as well.

Thanks

>
> So, it should be okay to remove a single mapping checking the others
> parameters (i.e. dir, size).
>
> I'll send a patch, so with the code it should be easier :-)
>
> Thanks,
> Stefano
>
>>
>>
>>>
>>> 2. There was a race between dma_map/unmap and the worker thread,
>>> since both are accessing the IOMMU. Taking the iommu_lock while
>>> using vhost_iotlb_* API in the worker thread fixes the "vringh:
>>> Failed to *" issues.
>>>
>>> Whit these issues fixed the vdpa-blk simulator seems to work well.
>>> I'll send the patches next week or after the break.
>>
>>
>> Good to know this.
>>
>> Thanks
>>
>>
>>>
>>> Thanks,
>>> Stefano
>>>
>>