linux-kernel - Re: [PATCH 0/2] KVM: MMU: support VMAs that got remap_pfn

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Date:	Mon, 4 Jul 2016 01:41:28 -0700
From:	Neo Jia <cjia@...dia.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
CC:	Paolo Bonzini <pbonzini@...hat.com>,
	<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	Kirti Wankhede <kwankhede@...dia.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH 0/2] KVM: MMU: support VMAs that got remap_pfn_range-ed

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.

Thanks,
Neo