[vfio-users] VFIO_IOMMU_MAP_DMA succeeds only on second try?

[Oliver Heid]

Many thanks!

mmap succeeds - I removed the checks only to post the code on this list. 
I use this now:

     if ( !ioctl(container, VFIO_IOMMU_MAP_DMA, &dma_map) )
     {    if ( !ioctl(container, VFIO_IOMMU_MAP_DMA, &dma_map) )
          {   perror("mmap ioctl"); return 1; }
     }

It mostly succeeds only on second try but I never needed more than two 
tries. So it's ok.

Re. IOMMU type 1 recognition: You are right: It is independent of kernel 
version. It seems to be related to auto-loading vfio_pci via 
/etc/modules.conf. Installing the module later gets it working.


------------------------------------------------------------------------

*From:* Alex Williamson <mailto:alex.williamson at redhat.com>
*Sent:* Monday, March 12, 2018 6:57AM
*To:* Heid, Oliver <mailto:Oliver.Heid at h-next.de>
*Cc:* Vfio-users <mailto:vfio-users at redhat.com>
*Subject:* Re: [vfio-users] VFIO_IOMMU_MAP_DMA succeeds only on second try?


On Mon, 5 Mar 2018 12:37:24 +0100
Oliver Heid <Oliver.Heid at h-next.de> wrote:

> Do I miss something here? I want to allow RW access of a peripheral
> device to a memory region via
>
>       struct vfio_iommu_type1_dma_map dma_map = { .argsz = sizeof(dma_map) };
>
>       __u32* mem = (__u32*)mmap(NULL,size,PROT_READ|PROT_WRITE,
> MAP_SHARED|MAP_ANONYMOUS|MAP_LOCKED, -1, 0);
>
>       dma_map.argsz = sizeof(dma_map);
>       dma_map.flags = VFIO_DMA_MAP_FLAG_READ | VFIO_DMA_MAP_FLAG_WRITE;
>       dma_map.vaddr = (__u64)mem;
>       dma_map.iova  = 0;
>       dma_map.size  = size;
>
>       ioctl(container, VFIO_IOMMU_MAP_DMA, &dma_map);
>
> Any idea why theVFIO_IOMMU_MAP_DMA ioctl fails with EINVAL, but then a
> second, identical call succeeds? Does it actually succeed then? QEMU
> re-tries if the first attempt fails with EBUSY, but not with EINVAL.

I don't think the QEMU -EBUSY behavior is related, that handles
previous mappings and may no longer even be necessary.  One of my unit
tests[1] does a similar map with no issues.  Your code excerpt doesn't
check whether mmap succeeds, so for all we know mmap is failing too.

> The latest 4.15.7 kernel (4.15.6 too IIRC) does not recognize the AMD
> Ryzen 3 1200 as  VFIO_TYPE1_IOMMU so I am using version 4.15.4.

This doesn't make sense, anything implementing the kernel's IOMMU API in
the kernel is compatible with type1, that includes anything supporting
AMD-Vi.  There is no whitelist or specific processor support
selection.  There are no iommu or vfio changes between the kernels
you're quoting, perhaps you could do a bisect.  Thanks,

Alex

[1] https://github.com/awilliam/tests/blob/master/vfio-iommu-map-unmap.c


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