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

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Message-ID: <20160704075302.GA13470@nvidia.com>
Date:	Mon, 4 Jul 2016 00:53:02 -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 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.

> 
> >
> >>
> >>Why the memory mapped by this mmap() is not a portion of MMIO from
> >>underlayer physical device? If it is a valid system memory, is this interface
> >>really needed to implemented in vfio? (you at least need to set VM_MIXEDMAP
> >>if it mixed system memory with MMIO)
> >>
> >
> >It actually is a portion of the physical mmio which is set by vfio mmap.
> 
> So i do not think we need to care its refcount, i,e, we can consider it as reserved_pfn,
> Paolo?
> 
> >
> >>IIUC, the kernel assumes that VM_PFNMAP is a continuous memory, e.g, like
> >>current KVM and vaddr_get_pfn() in vfio, but it seems nvdia's patchset
> >>breaks this semantic as ops->validate_map_request() can adjust the physical
> >>address arbitrarily. (again, the name 'validate' should be changed to match
> >>the thing as it is really doing)
> >
> >The vgpu api will allow you to adjust the target mmio address and the size via
> >validate_map_request, but it is still physical contiguous as <start_pfn, size>.
> 
> Okay, the interface confused us, maybe this interface need to be cooked to reflect
> to this fact.

Sure. We can address this in the RFC mediated device thread.

Thanks,
Neo

> 
> Thanks!
>