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

[Haozhong Zhang <haozhong.zhang@intel.com>]

On 01/16/17 11:00 +0000, Stefan Hajnoczi wrote:
>On Mon, Jan 16, 2017 at 01:55:34PM +0800, Xiao Guangrong wrote:
>> On 01/14/2017 02:02 AM, Eduardo Habkost wrote:
>> > 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 <stefanha@redhat.com>
>> > > > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
>> > > > Message-Id: 20170112110928.GF4621@stefanha-x1.localdomain
>> > > > Message-Id: 20170111093630.2088-1-stefanha@redhat.com
>> > > > ---
>> > > >  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?
>> >
>>
>> Yes. i agree with Eduardo.
>>
>> For the real hardware the behavior may be different between vendors, we
>> are asking our HW people to check what will happen on Intel's hardware
>> under this case.
>
>Let's find out what real hardware/firmware does.  I guess it's the Intel
>MRC component that handles memory initialization.
>

The behavior of NVDIMM on unsupported platform (HW/FW) is vendor
specific. For some vendors, it's undefined and the platform may do
anything (e.g. the three points Stefan listed above). Thus, I think
QEMU is free to choose the implementation. Aborting QEMU
(i.e. refusing to boot) is the easiest one.

Haozhong