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

[Xiao Guangrong <guangrong.xiao@linux.intel.com>]

On 07/04/2016 03:53 PM, Neo Jia wrote:
> On Mon, Jul 04, 2016 at 03:37:35PM +0800, Xiao Guangrong wrote:
>>
>>
>> On 07/04/2016 03:03 PM, Neo Jia wrote:
>>> On Mon, Jul 04, 2016 at 02:39:22PM +0800, Xiao Guangrong wrote:
>>>>
>>>>
>>>> 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?
>>>
>>> Hi Guangrong,
>>>
>>> as such mapping information between virtual mmio to physical mmio is only available
>>> at runtime.
>>
>> Sorry, i do not know what the different between mmap() and the time VM actually
>> accesses the memory for your case. Could you please more detail?
>
> Hi Guangrong,
>
> Sure. The mmap() gets called by qemu or any VFIO API userspace consumer when
> setting up the virtual mmio, at that moment nobody has any knowledge about how
> the physical mmio gets virtualized.
>
> When the vm (or application if we don't want to limit ourselves to vmm term)
> starts, the virtual and physical mmio gets mapped by mpci kernel module with the
> help from vendor supplied mediated host driver according to the hw resource
> assigned to this vm / application.

Thanks for your expiation.

It sounds like a strategy of resource allocation, you delay the allocation until VM really
accesses it, right?