Re: [Qemu-devel] [PATCH v2 3/4] nvdimm: add a boolean option "restrict"

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Jun 08, 2017 at 03:56:42PM +0300, Michael S. Tsirkin wrote:
> On Tue, Jun 06, 2017 at 03:22:28PM +0800, Haozhong Zhang wrote:
> > If a vNVDIMM device is not backed by a DAX device and its "restrict"
> > option is enabled, bit 3 of state flags in its region mapping
> > structure will be set, in order to notify the guest of the lack of
> > write persistence guarantee. Once this bit is set, the guest OS may
> > mark the vNVDIMM device as read-only.
> > 
> > This option is disabled by default for backwards compatibility. It's
> > recommended to enable for the formal usage.
> > 
> > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> 
> Seems wrong to me. E.g. it won't work in a nested
> virt setup. What if backend is dax but is not armed?
> Can't the armed bit of the backing device be tested?
> Name "restrict" is also confusing. Can we reuse cache=
> options? E.g. cache=unsafe etc.

The -drive cache= options (writeback, writethrough, none, directsync,
unsafe) are confusing and considered legacy options.  The new options
are -drive
cache.writeback=on|off,cache.direct=on|off,cache.no-flush=on|off.

I suggested to call the option -device nvdimm,armed=auto|on|off in
another email.  "Armed" is the term used by the NVDIMM/NFIT
specification and it has an NVDIMM-specific meaning.

Stefan

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