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

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Date:	Wed, 6 Jul 2016 10:35:18 +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 07/06/2016 10:18 AM, Neo Jia wrote:
> On Wed, Jul 06, 2016 at 10:00:46AM +0800, Xiao Guangrong wrote:
>>
>>
>> On 07/05/2016 08:18 PM, Paolo Bonzini wrote:
>>>
>>>
>>> On 05/07/2016 07:41, Neo Jia wrote:
>>>> On Thu, Jun 30, 2016 at 03:01:49PM +0200, 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.
>>>>>
>>>>> KVM lacks support for this kind of non-linear VM_PFNMAP mapping, and these
>>>>> patches should fix this.
>>>>
>>>> Hi Paolo,
>>>>
>>>> I have tested your patches with the mediated passthru patchset that is being
>>>> reviewed in KVM and QEMU mailing list.
>>>>
>>>> The fault handler gets called successfully and the previously mapped memory gets
>>>> unmmaped correctly via unmap_mapping_range.
>>>
>>> Great, then I'll include them in 4.8.
>>
>> Code is okay, but i still suspect if this implementation, fetch mmio pages in fault
>> handler, is needed. We'd better include these patches after the design of vfio
>> framework is decided.
>
> Hi Guangrong,
>
> I disagree. The design of VFIO framework has been actively discussed in the KVM
> and QEMU mailing for a while and the fault handler is agreed upon to provide the
> flexibility for different driver vendors' implementation. With that said, I am
> still open to discuss with you and anybody else about this framework as the goal
> is to allow multiple vendor to plugin into this framework to support their
> mediated device virtualization scheme, such as Intel, IBM and us.

The discussion is still going on. And current vfio patchset we reviewed is still
problematic.

>
> May I ask you what the exact issue you have with this interface for Intel to support
> your own GPU virtualization?

Intel's vGPU can work with this framework. We really appreciate your / nvidia's
contribution.

i didn’t mean to offend you, i just want to make sure if this complexity is really
needed and inspect if this framework is safe enough and think it over if we have
a better implementation.