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Re: [Qemu-devel] [PATCH] hw/i386: check if nvdimm is enabled before plug


From: Eduardo Habkost
Subject: Re: [Qemu-devel] [PATCH] hw/i386: check if nvdimm is enabled before plugging
Date: Fri, 13 Jan 2017 16:02:43 -0200
User-agent: Mutt/1.7.1 (2016-10-04)

On Fri, Jan 13, 2017 at 01:17:27PM +0000, Stefan Hajnoczi wrote:
> On Fri, Jan 13, 2017 at 07:56:51PM +0800, Haozhong Zhang wrote:
> > The missing of 'nvdimm' in the machine type option '-M' means NVDIMM
> > is disabled. QEMU should refuse to plug any NVDIMM device in this case
> > and report the misconfiguration.
> > 
> > Reported-by: Stefan Hajnoczi <address@hidden>
> > Signed-off-by: Haozhong Zhang <address@hidden>
> > Message-Id: address@hidden
> > Message-Id: address@hidden
> > ---
> >  hw/i386/pc.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > index 25e8586..3907609 100644
> > --- a/hw/i386/pc.c
> > +++ b/hw/i386/pc.c
> > @@ -1715,6 +1715,11 @@ static void pc_dimm_plug(HotplugHandler *hotplug_dev,
> >      }
> >  
> >      if (object_dynamic_cast(OBJECT(dev), TYPE_NVDIMM)) {
> > +        if (!pcms->acpi_nvdimm_state.is_enabled) {
> > +            error_setg(&local_err,
> > +                       "nvdimm is not enabled: missing 'nvdimm' in '-M'");
> > +            goto out;
> > +        }
> 
> A warning is definitely useful to notify users of a possible
> configuration error.
> 
> I wonder what happens when you plug an NVDIMM into a motherboard where
> the firmware lacks support.  Does it:
>  * Refuse to boot?
>  * Treat the DIMM as regular RAM?
>  * Boot but the DIMM will not be used by firmware and kernel?
> 
> QEMU should act the same way as real hardware.

If real hardware behavior is not useful in any way (e.g. first
and third options above), is there a good reason for QEMU to not
implement an additional safety mechanism preventing NVDIMM from
being connected to a machine that doesn't support it?

-- 
Eduardo



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