Re: [Qemu-devel] [RESEND PATCH 1/2] nvdimm: warn if the backend is not a DAX device

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, May 25, 2017 at 08:34:23PM -0700, Dan Williams wrote:
> On Thu, May 25, 2017 at 7:32 PM, Haozhong Zhang
> <haozhong.zhang@intel.com> wrote:
> > Applications in Linux guest that use device-dax never trigger flush
> > that can be trapped by KVM/QEMU. Meanwhile, if the host backend is not
> > device-dax, QEMU cannot guarantee the persistence of guest writes.
> > Before solving this flushing problem, QEMU should warn users if the
> > host backend is not device-dax.
> 
> I think this needs to be stronger than a "warn" it needs to be
> explicitly forbidden when it is known to be unsafe.

I think users should have the choice in what they want to do -
QEMU should not artifically block it.  There are plenty of things
in QEMU that are potentially unsafe in some usage scenarios, but
we just document how to use them in a safe manner & any caveats
that apply. Higher level applications above QEMU can then consider
how they want to apply a usage policy to meet the needs of their
usage scenario.

Having an emulated DAX device that doesn't guarantee persistence
is no different to having an emulated disk device that never flushes
to underlying host storage.

Regards,
Daniel
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