Hi, Jia, On Thu, Nov 19, 2020 at 10:27:37PM +0800, Jia He wrote: > The permission of vfio iommu is different and incompatible with vma > permission. If the iotlb->perm is IOMMU_NONE (e.g. qemu side), qemu will > simply call unmap ioctl() instead of mapping. Hence vfio_dma_map() can't > map a dma region with NONE permission. > > This corner case will be exposed in coming virtio_fs cache_size > commit [1] > - mmap(NULL, size, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); > memory_region_init_ram_ptr() > - re-mmap the above area with read/write authority. If iiuc here we'll remap the above PROT_NONE into PROT_READ|PROT_WRITE, then... > - vfio_dma_map() will be invoked when vfio device is hotplug added. ... here I'm slightly confused on why VFIO_IOMMU_MAP_DMA would encounter vma check fail - aren't they already get rw permissions? I'd appreciate if you could explain why vfio needs to dma map some PROT_NONE pages after all, and whether QEMU would be able to postpone the vfio map of those PROT_NONE pages until they got to become with RW permissions. Thanks, -- Peter Xu
