Re: [RFC 00/18] vfio: Adopt iommufd

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 26 Apr 2022 12:43:35 +0000
Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com> wrote:

> > -----Original Message-----
> > From: Eric Auger [mailto:eric.auger@redhat.com]
> > Sent: 26 April 2022 12:45
> > To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>; Yi
> > Liu <yi.l.liu@intel.com>; alex.williamson@redhat.com; cohuck@redhat.com;
> > qemu-devel@nongnu.org
> > Cc: david@gibson.dropbear.id.au; thuth@redhat.com; farman@linux.ibm.com;
> > mjrosato@linux.ibm.com; akrowiak@linux.ibm.com; pasic@linux.ibm.com;
> > jjherne@linux.ibm.com; jasowang@redhat.com; kvm@vger.kernel.org;
> > jgg@nvidia.com; nicolinc@nvidia.com; eric.auger.pro@gmail.com;
> > kevin.tian@intel.com; chao.p.peng@intel.com; yi.y.sun@intel.com;
> > peterx@redhat.com; Zhangfei Gao <zhangfei.gao@linaro.org>
> > Subject: Re: [RFC 00/18] vfio: Adopt iommufd  
> 
> [...]
>  
> > >>  
> > https://lore.kernel.org/kvm/0-v1-e79cd8d168e8+6-iommufd_jgg@nvidia.com  
> > >> /
> > >> [2] https://github.com/luxis1999/iommufd/tree/iommufd-v5.17-rc6
> > >> [3] https://github.com/luxis1999/qemu/tree/qemu-for-5.17-rc6-vm-rfcv1  
> > > Hi,
> > >
> > > I had a go with the above branches on our ARM64 platform trying to  
> > pass-through  
> > > a VF dev, but Qemu reports an error as below,
> > >
> > > [    0.444728] hisi_sec2 0000:00:01.0: enabling device (0000 -> 0002)
> > > qemu-system-aarch64-iommufd: IOMMU_IOAS_MAP failed: Bad address
> > > qemu-system-aarch64-iommufd: vfio_container_dma_map(0xaaaafeb40ce0,  
> > 0x8000000000, 0x10000, 0xffffb40ef000) = -14 (Bad address)  
> > >
> > > I think this happens for the dev BAR addr range. I haven't debugged the  
> > kernel  
> > > yet to see where it actually reports that.  
> > Does it prevent your assigned device from working? I have such errors
> > too but this is a known issue. This is due to the fact P2P DMA is not
> > supported yet.
> >   
> 
> Yes, the basic tests all good so far. I am still not very clear how it works if
> the map() fails though. It looks like it fails in,
> 
> iommufd_ioas_map()
>   iopt_map_user_pages()
>    iopt_map_pages()
>    ..
>      pfn_reader_pin_pages()
> 
> So does it mean it just works because the page is resident()?

No, it just means that you're not triggering any accesses that require
peer-to-peer DMA support.  Any sort of test where the device is only
performing DMA to guest RAM, which is by far the standard use case,
will work fine.  This also doesn't affect vCPU access to BAR space.
It's only a failure of the mappings of the BAR space into the IOAS,
which is only used when a device tries to directly target another
device's BAR space via DMA.  Thanks,

Alex