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Re: [PATCH] spapr_nvdimm.c: make 'label-size' mandatory

From: Daniel Henrique Barboza
Subject: Re: [PATCH] spapr_nvdimm.c: make 'label-size' mandatory
Date: Tue, 14 Apr 2020 08:04:29 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0

On 4/13/20 11:01 PM, David Gibson wrote:
CCing Xiao, Michael and Igor for generic NVDIMM perspective.

On Mon, Apr 13, 2020 at 05:36:28PM -0300, Daniel Henrique Barboza wrote:
The pseries machine does not support NVDIMM modules without label.
Attempting to do so, even if the overall block size is aligned with
256MB, will seg fault the guest kernel during NVDIMM probe. This
can be avoided by forcing 'label-size' to always be present for

The verification was put before the alignment check because the
presence of label-size affects the alignment calculation, so
it's not optimal to warn the user about an alignment error,
then about the lack of label-size, then about a new alignment
error when the user sets a label-size.

Signed-off-by: Daniel Henrique Barboza <address@hidden>

So, this would get the job done, but it seems a bit inelegant compared
to having the device default to working settings.  I'm looking at how
this interacts with the generic constraints on label-size.

The generic nvdimm code has a MIN_NAMESPACE_LABEL_SIZE of 128 kiB, and
values of label-size less than that are rejected.  Except that if
label-size is not set at all, it is left as 0.

Is that intended behaviour?  Do x86 (or whatever) NVDIMMs have a label
of at least 128kiB, unless they have no label at all?  Or could we
make the default label-size 128kiB generically?

My limited understanding on how NVDIMM works in x86 is that x86 NVDIMMs can
work with and without label, but the label has a minimum size of 128kiB.

The current NVDIMM init code always adds NVDIMM_LABEL_SIZE_PROP regardless
of user settings, as you can see in hw/mem/nvdimm.c, nvdimm_init(). I found
this out in my first attempt in this patch when doing

object_property_find(OBJECT(nvdimm), NVDIMM_LABEL_SIZE_PROP,
                     &error_abort) == 0) == NULL

inside spapr_nvdimm.c and finding out that this property is always present. This
means that the way to distinguish between label and label-less NVDIMMs is to
check label_size != 0, as we can see being done in hw/acpi/nvdimm.c. What I did
then was to use this same approach in spapr_nvdimm.c.



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