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

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Date:	Wed, 6 Jul 2016 09:50:50 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	Neo Jia <cjia@...dia.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, 6 Jul 2016 08:05:15 +0200
Paolo Bonzini <pbonzini@...hat.com> wrote:

> On 06/07/2016 04:00, 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.  
> 
> I think that this fixes a bug anyway, the previous handling of VM_PFNMAP
> is too simplistic.


Agreed, no reason to hold off on this, it's a valid interaction that
needs to be fixed regardless of how or if the vfio mediated driver
makes use of it.  Thanks,

Alex