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Re: Two questions about NVDIMM devices



On 9/10/20 4:56 PM, Daniel P. Berrangé wrote:
On Thu, Sep 10, 2020 at 04:54:08PM +0200, Milan Zamazal wrote:
Daniel P. Berrangé <berrange redhat com> writes:

On Thu, Sep 10, 2020 at 04:26:40PM +0200, Milan Zamazal wrote:
Daniel P. Berrangé <berrange redhat com> writes:


On Thu, Jul 02, 2020 at 01:21:15PM +0200, Milan Zamazal wrote:
Hi,


I've met two situations with NVDIMM support in libvirt where I'm not
sure all the parties (libvirt & I) do the things correctly.

The first problem is with memory alignment and size changes.  In
addition to the size changes applied to NVDIMMs by QEMU, libvirt also
makes some NVDIMM size changes for better alignments, in
qemuDomainMemoryDeviceAlignSize.  This can lead to the size being
rounded up, exceeding the size of the backing device and QEMU failing to
start the VM for that reason (I've experienced that actually).  I work
with emulated NVDIMM devices, not a bare metal hardware, so one might
argue that in practice the device sizes should already be aligned, but
I'm not sure it must be always the case considering labels or whatever
else the user decides to set up.  And I still don't feel very
comfortable that I have to count with two internal size adjustments
(libvirt & QEMU) to the `size' value I specify, with the ultimate goal
of getting the VM started and having the NVDIMM aligned properly to make
(non-NVDIMM) memory hot plug working.  Is the size alignment performed
by libvirt, especially rounding up, completely correct for NVDIMMs?

The comment on the function says QEMU aligns to "page size", which
is something that can vary depending not only on architecture, and
also the build config options for the kernel on that architecture.
eg aarch64 has different page size in RHEL than other distros because
of different choice of page size in kernel config.

Libvirt rounds up to 1 MB, essentially so that the size works no matter
what architecture or build options were used. I think this is quite
compelling as I don't think mgmt apps are likely to care enough about
non-x86 architectures to pick the right rounded sizes.

If we're enforcing this 1 MB rounding though, we really should be
documenting it clearly, so that apps can pick the right backing file
size. I think we dropped the ball on docs.

I still can't see it in the documentation, would it be possible to be
clear about it in the docs, please?  For first, it's not very intuitive
to figure out that (if I've figured out it correctly) on POWER one
*must* specify the NVDIMM size S as

   S == aligned_size + label_size

and that size is used for the QEMU device; while on x86_64 one can
specify any size S and

   align_up(S)

will be used for the QEMU device (and label size doesn't influence the
value).  And additional alignment may be required for having any memory
hot plug working.

For second, and more importantly, I'm afraid that without documenting
it, future changes may break the current behavior without warning.  For
example, the recent changes regarding POWER alignment in 6.7.0 are for
good IMHO and one can use the same size with both 6.7 and 6.6 versions,
but they could still cause pre-6.7 sizes stop working.

I don't know what changes you are referring to here, but if they were
in libvirt I'd consider that a bug - we shouldn't break a previously
working configuration by increasing required alignment.

I mean disabling the auto alignment in
https://gitlab.com/libvirt/libvirt/-/commit/07de813924caf37e535855541c0c1183d9d382e2
and replacing it with validation in
https://gitlab.com/libvirt/libvirt/-/commit/0ccceaa57c50e5ee528f7073fa8723afd62b88b7

That change can cause a VM fail to start but after (manually) adjusting
the device size, all should work all right.  Changes that would actually
change sizes would be more dangerous.

Sigh, that second commit even calls out the fact that it breaks
existing guests. This needs to be reverted, as that is not acceptable.

Thing is, on PPC it was never working IIRC. I remember discussing this with Andrea. So from my POV, there wasn't really anything to break.

Michal


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