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

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Date:	Mon, 4 Jul 2016 08:33:03 -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 06:16:46PM +0800, Xiao Guangrong wrote:
> 
> 
> On 07/04/2016 05:16 PM, Neo Jia wrote:
> >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.
> 
> So how to understand your statement:
> "at that moment nobody has any knowledge about how the physical mmio gets virtualized"
> 
> The resource, physical MMIO region, has been allocated, why we do not know the physical
> address mapped to the VM?
> 

>From a device driver point of view, the physical mmio region never gets allocated until 
the corresponding resource is requested by clients and granted by the mediated device driver. 

The resource here is the internal hw resource.

"at that moment" == vfio client triggers mmap() call.

Thanks,
Neo

> 
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