On Thu, Aug 20, 2009 at 12:14 AM, Avi Kivity<avi@xxxxxxxxxx> wrote: > On 08/13/2009 07:07 AM, Stephen Donnelly wrote: >> >> npages = get_user_pages_fast(addr, 1, 1, page); returns -EFAULT, >> presumably because (vma->vm_flags& (VM_IO | VM_PFNMAP)). >> >> It takes then unlikely branch, and checks the vma, but I don't >> understand what it is doing here: pfn = ((addr - vma->vm_start)>> >> PAGE_SHIFT) + vma->vm_pgoff; > > It's calculating the pfn according to pfnmap rules. >From what I understand this will only work when remapping 'main memory', e.g. where the pgoff is equal to the physical page offset? VMAs that remap IO memory will usually set pgoff to 0 for the start of the mapping. >> In my case addr == vma->vm_start, and vma->vm_pgoff == 0, so pfn ==0. > > How did you set up that vma? It should point to the first pfn of your > special memory area. The vma was created with a remap_pfn_range call from another driver. Because this call sets VM_PFNMAP and VM_IO any get_user_pages(_fast) calls will fail. In this case the host driver was actually just remapping host memory, so I replaced the remap_pfn_range call with a nopage/fault vm_op. This allows the get_user_pages_fast call to succeed, and the mapping now works as expected. This is sufficient for my work at the moment. I'm still not sure how genuine IO memory (mapped from a driver to userspace with remap_pfn_range or io_remap_page_range) could be mapped into kvm though. Regards, Stephen. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
