Date:   Mon, 21 Dec 2020 12:14:23 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Jason Wang <jasowang@...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 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.

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
>>
>

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