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

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Date:	Mon, 4 Jul 2016 02:16:09 -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 04:45:05PM +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

> 
> 
>