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

[Stefano Garzarella]

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