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    SubjectRe: [PATCH 0/2] KVM: MMU: support VMAs that got remap_pfn_range-ed
    On Mon, Jul 04, 2016 at 04:45:05PM +0800, Xiao Guangrong wrote:
    >
    >
    > On 07/04/2016 04:41 PM, Neo Jia wrote:
    > >On Mon, Jul 04, 2016 at 04:19:20PM +0800, Xiao Guangrong wrote:
    > >>
    > >>
    > >>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?
    > >
    > >Yes, that is where the fault handler inside mpci code comes to the picture.
    >
    >
    > I am not sure this strategy is good. The instance is successfully created, and it is started
    > successful, but the VM is crashed due to the resource of that instance is not enough. That sounds
    > unreasonable.


    Sorry, I think I misread the "allocation" as "mapping". We only delay the
    cpu mapping, not the allocation.

    Thanks,
    Neo

    >
    >
    >

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