Re: [Qemu-devel] [RFC QEMU PATCH v4 00/10] Implement vNVDIMM for Xen HVM guest

[Anthony PERARD]

On Wed, Feb 28, 2018 at 05:36:59PM +0800, Haozhong Zhang wrote:
> On 02/27/18 17:22 +0000, Anthony PERARD wrote:
> > On Thu, Dec 07, 2017 at 06:18:02PM +0800, Haozhong Zhang wrote:
> > > This is the QEMU part patches that works with the associated Xen
> > > patches to enable vNVDIMM support for Xen HVM domains. Xen relies on
> > > QEMU to build guest NFIT and NVDIMM namespace devices, and allocate
> > > guest address space for vNVDIMM devices.
> > 
> > I've got other question, and maybe possible improvements.
> > 
> > When QEMU build the ACPI tables, it also initialize some MemoryRegion,
> > which use more guest memory. Do you know if those regions are used with
> > your patch series on Xen?
> 
> Yes, that's why dm_acpi_size is introduced.
> 
> > Otherwise, we could try to avoid their
> > creation with this:
> > In xenfv_machine_options()
> > m->rom_file_has_mr = false;
> > (setting this in xen_hvm_init() would probably be better, but I havn't
> > try)
> 
> If my memory is correct, simply setting rom_file_has_mr to false does
> not work (though I cannot remind the exact reason). I'll have a look
> as the code to refresh my memory.

I've played a bit with this idea, but without a proper NVDIMM available
for the guest, so I don't know if it's going to work properly without
the mr.

To make it work, I had to disable some code in acpi_build_update() that
make use of the MemoryRegions, as well as an assert in acpi_setup().
After those small hacks, I could boot the guest, and I've check that the
expected ACPI tables where there, and they looked correct to my eyes.
And least `ndctl list` works and showed the nvdimm (that I have
configured on QEMU's cmdline).

But I may not have been far enough with my tests, and maybe something
later relies on the MRs, especially the _DSM method that I don't know if
it was working properly.

Anyway, that why I proposed the idea, and if we can avoid more
uncertainty about how much guest memory QEMU is going to use, that would
be good.

Thanks,

-- 
Anthony PERARD