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

[Haozhong Zhang]

On 05/26/17 08:25 -0700, Dan Williams wrote:
> On Fri, May 26, 2017 at 7:38 AM, Daniel P. Berrange <address@hidden> 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
> >> <address@hidden> 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.
> 
> In other words, I agree that it should be possible to tell the guest
> to assume it is pmem when it is not, but we need more granularity in
> the configuration to communicate the capabilities correctly to the
> guest. It seems the NFIT memory device state flag
> ACPI_NFIT_MEM_NOT_ARMED is a good way to communicate pmem safety to
> the guest. Can we add a new knob to control the polarity of that flag
> and make ACPI_NFIT_MEM_NOT_ARMED being set the default case when using
> regular file mmap and clear the flag by default in the device-dax
> case?

Yes, we can set ACPI_NFIT_MEM_NOT_ARMED if the host backend is not
device-dax.

Thanks,
Haozhong