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

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Message-ID: <cc913954-135d-da5a-1acd-38c3ecb77dfa@redhat.com>
Date:   Tue, 22 Dec 2020 20:29:20 +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 2020/12/22 下午6:57, Stefano Garzarella wrote:
> 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 :-)


Kernel had implemented one in iova.c but I'm not sure we need the 
complexity like that. Or we can just use rbtree or idr to implement a 
simpler one.

Thanks


>
> Thanks,
> Stefano
>