Re: [Xen-devel] [RFC Design Doc] Add vNVDIMM support for Xen

[Haozhong Zhang <haozhong.zhang@xxxxxxxxx>]

On 03/17/16 14:00, Ian Jackson wrote:
> Haozhong Zhang writes ("Re: [Xen-devel] [RFC Design Doc] Add vNVDIMM support 
> for Xen"):
> > QEMU keeps mappings of guest memory because (1) that mapping is
> > created by itself, and/or (2) certain device emulation needs to access
> > the guest memory. But for vNVDIMM, I'm going to move the creation of
> > its mappings out of qemu to toolstack and vNVDIMM in QEMU does not
> > access vNVDIMM pages mapped to guest, so it's not necessary to let
> > qemu keeps vNVDIMM mappings.
> 
> I'm confused by this.
> 
> Suppose a guest uses an emulated device (or backend) provided by qemu,
> to do DMA to an vNVDIMM.  Then qemu will need to map the real NVDIMM
> pages into its own address space, so that it can write to the memory
> (ie, do the virtual DMA).
> 
> That virtual DMA might well involve a direct mapping in the kernel
> underlying qemu: ie, qemu might use O_DIRECT to have its kernel write
> directly to the NVDIMM, and with luck the actual device backing the
> virtual device will be able to DMA to the NVDIMM.
> 
> All of this seems to me to mean that qemu needs to be able to map
> its guest's parts of NVDIMMs
> 
> There are probably other example: memory inspection systems used by
> virus scanners etc.; debuggers used to inspect a guest from outside;
> etc.
> 
> I haven't even got started on save/restore...
> 

Oops, so many cases I missed. Thanks Ian for pointing out all these!
Now I need to reconsider how to manage guest permissions for NVDIMM pages.

Haozhong

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