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

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

On Fri, May 26, 2017 at 08:25:20AM -0700, Dan Williams wrote:
> On Fri, May 26, 2017 at 7:38 AM, Daniel P. Berrange <berrange@redhat.com> wrote:
> > 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.
> >
> 
> It is different in the sense that the contract of when the guest
> should assume persistence is specified by when the write completes to
> the virtual disk. In the case of the virtual NFIT we are currently
> lying to the guest about that platform persistence guarantee even if
> the hypervisor is emulating pmem with volatile memory.

We equally lie to the guest about persistence of disks, when the
disk is run with certain cache= settings, or when the disk backing file
is on tmpfs. It is simply a choice the mgmt application makes about
whether to provide backing storage that is capable of satsifying the
guarantees implied by the guest device model. So I'm still not seeing
a compelling reason to artifically block this with DAX.

Regards,
Daniel
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