Re: [PATCH 0/2] KVM: MMU: support VMAs that got remap_pfn_range-ed

[Xiao Guangrong]

On 06/30/2016 09:01 PM, Paolo Bonzini wrote:
> The vGPU folks would like to trap the first access to a BAR by setting
> vm_ops on the VMAs produced by mmap-ing a VFIO device.  The fault handler
> then can use remap_pfn_range to place some non-reserved pages in the VMA.

Why does it require fetching the pfn when the fault is triggered rather
than when mmap() is called?

Why the memory mapped by this mmap() is not a portion of MMIO from
underlayer physical device? If it is a valid system memory, is this interface
really needed to implemented in vfio? (you at least need to set VM_MIXEDMAP
if it mixed system memory with MMIO)

IIUC, the kernel assumes that VM_PFNMAP is a continuous memory, e.g, like
current KVM and vaddr_get_pfn() in vfio, but it seems nvdia's patchset
breaks this semantic as ops->validate_map_request() can adjust the physical
address arbitrarily. (again, the name 'validate' should be changed to match
the thing as it is really doing)