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

From: Xiao Guangrong
Subject: Re: [Qemu-devel] [PATCH] hw/i386: check if nvdimm is enabled before plugging
Date: Mon, 16 Jan 2017 13:55:34 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

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 <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?

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.


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