Re: [PATCH v1: vfio: avoid unnecessary pin memory when dma map io address space 0/2]

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Oct 24, 2024 at 11:06:24AM -0600, Alex Williamson wrote:
> On Thu, 24 Oct 2024 17:34:42 +0800
> Qinyun Tan <qinyuntan@linux.alibaba.com> wrote:
> 
> > When user application call ioctl(VFIO_IOMMU_MAP_DMA) to map a dma address,
> > the general handler 'vfio_pin_map_dma' attempts to pin the memory and
> > then create the mapping in the iommu.
> > 
> > However, some mappings aren't backed by a struct page, for example an
> > mmap'd MMIO range for our own or another device. In this scenario, a vma
> > with flag VM_IO | VM_PFNMAP, the pin operation will fail. Moreover, the
> > pin operation incurs a large overhead which will result in a longer
> > startup time for the VM. We don't actually need a pin in this scenario.
> > 
> > To address this issue, we introduce a new DMA MAP flag
> > 'VFIO_DMA_MAP_FLAG_MMIO_DONT_PIN' to skip the 'vfio_pin_pages_remote'
> > operation in the DMA map process for mmio memory. Additionally, we add
> > the 'VM_PGOFF_IS_PFN' flag for vfio_pci_mmap address, ensuring that we can
> > directly obtain the pfn through vma->vm_pgoff.
> > 
> > This approach allows us to avoid unnecessary memory pinning operations,
> > which would otherwise introduce additional overhead during DMA mapping.
> > 
> > In my tests, using vfio to pass through an 8-card AMD GPU which with a
> > large bar size (128GB*8), the time mapping the 192GB*8 bar was reduced
> > from about 50.79s to 1.57s.
> 
> If the vma has a flag to indicate pfnmap, why does the user need to
> provide a mapping flag to indicate not to pin?  We generally cannot
> trust such a user directive anyway, nor do we in this series, so it all
> seems rather redundant.

The best answer is to map from DMABUF not from VMA and then you get
perfect aggregation cheaply.
 
> What about simply improving the batching of pfnmap ranges rather than
> imposing any sort of mm or uapi changes?  Or perhaps, since we're now
> using huge_fault to populate the vma, maybe we can iterate at PMD or
> PUD granularity rather than PAGE_SIZE?  Seems like we have plenty of
> optimizations to pursue that could be done transparently to the
> user.

I don't want to add more stuff to support the security broken
follow_pfn path. It needs to be replaced.

Leon's work to improve the DMA API is soo close so we may be close to
the end!

There are two versions of the dmabuf patches on the list, it would be
good to get that in good shape. We could make a full solution,
including the vfio/iommufd map side while waiting.

Jason