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

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Date:   Tue, 22 Dec 2020 11:57:33 +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 Tue, Dec 22, 2020 at 10:44:48AM +0800, Jason Wang wrote:
>
>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.

Yes, sorry, I didn't explain well previously.

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

Okay, I'll work on it.
If you have an example to follow or some pointers, they are welcome :-)

Thanks,
Stefano