On 06/11/18 19:55, Dan Williams wrote: > On Mon, Jun 11, 2018 at 9:26 AM, Stefan Hajnoczi <stefa...@redhat.com> wrote: > > On Mon, Jun 11, 2018 at 06:54:25PM +0800, Zhang Yi wrote: > >> Nvdimm driver use Memory hot-plug APIs to map it's pmem resource, > >> which at a section granularity. > >> > >> When QEMU emulated the vNVDIMM device, decrease the label-storage, > >> QEMU will put the vNVDIMMs directly next to one another in physical > >> address space, which means that the boundary between them won't > >> align to the 128 MB memory section size. > > > > I'm having a hard time parsing this. > > > > Where does the "128 MB memory section size" come from? ACPI? > > A chipset-specific value? > > > > The devm_memremap_pages() implementation use the memory hotplug core > to allocate the 'struct page' array/map for persistent memory. Memory > hotplug can only be performed in terms of sections, 128MB on x86_64.
IIUC, it also affects the normal RAM hotplug to a Linux VM on QEMU. If that is the case, it will be helpful to lift this option to pc-dimm. Thanks, Haozhong > There is some limited support for allowing devm_memremap_pages() to > overlap 'System RAM' within a given section, but it does not currently > support multiple devm_memremap_pages() calls overlapping within the > same section. There is currently a kernel bug where we do not handle > this unsupported configuration gracefully. The fix will cause > configurations configurations that try to overlap 2 persistent memory > ranges in the same section to fail. > > The proposed fix is trying to make sure that QEMU does not run afoul > of this constraint. > > There is currently no line of sight to reduce the minimum memory > hotplug alignment size to less than 128M. Also, as other architectures > outside of x86_64 add devm_memremap_pages() support, the minimum > section alignment constraint might change and is a property of a guest > OS. My understanding is that some guest OSes might expect an even > larger persistent memory minimum alignment. >
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