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

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

On 03/02/18 12:03 +0000, Anthony PERARD wrote:
> 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.
> 

Yes, I also tested some non-trivial _DSM methods and it looks rom
files without memory regions can work with Xen after some
modifications. I'll apply this idea in the next version if no other
issues are found.

Thanks,
Haozhong