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

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Date:	Tue, 5 Jul 2016 19:57:50 -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 Wed, Jul 06, 2016 at 10:35:18AM +0800, Xiao Guangrong wrote:
> 
> 
> 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.

My point is the fault handler part has been discussed already, with that said I
am always open to any constructive suggestions to make things better and
maintainable. (Appreciate your code review on the VFIO thread, I think we still
own you another response, will do that.)

> 
> >
> >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.

Then, I don't think we should embargo Paolo's patch.

> 
> 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.

Not at all. :-)

Suggestions are always welcome, I just want to know the exact issues you have
with the code so I can have a better response to address that with proper 
information.

Thanks,
Neo