Re: [Qemu-devel] [RFC PATCH 1/1] nvdimm: let qemu requiring section alignment of pmem resource.

[Haozhong Zhang]

On 06/11/18 19:55, Dan Williams wrote:
> On Mon, Jun 11, 2018 at 9:26 AM, Stefan Hajnoczi <address@hidden> 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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