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

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Date:	Mon, 4 Jul 2016 16:54:25 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Neo Jia <cjia@...dia.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 07/04/2016 04:45 PM, 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.
>

Especially, you can not squeeze this kind of memory to balance the usage between all VMs. Does
this strategy still make sense?