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

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Date:   Mon, 21 Dec 2020 11:16:54 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Laurent Vivier <lvivier@...hat.com>,
        linux-kernel@...r.kernel.org, Eli Cohen <elic@...dia.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Max Gurtovoy <mgurtovoy@...dia.com>
Subject: Re: [PATCH RFC 00/12] vdpa: generalize vdpa simulator and add block
 device


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.


>
> 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
>