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

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Message-ID: <0a3c1304-6ed3-d68d-848c-1c9871f8dd53@redhat.com>
Date:   Tue, 22 Dec 2020 20:56:16 +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8:29, Jason Wang wrote:
>
> 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.


VDUSE[1] implements another allocator, but it's still complicated since 
it needs to track bounce pages. I feel like we'd better start from a 
simple one.

Thanks

[1] https://www.spinics.net/lists/linux-mm/msg231576.html

>
> Thanks
>
>
>>
>> Thanks,
>> Stefano
>>