Re: [Qemu-devel] [RFC v3 06/15] hw/arm/virt: Allocate device_memory

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 8 Aug 2018 11:33:23 +0200
Auger Eric <eric.auger@redhat.com> wrote:

> Hi Igor,
> 
> On 07/18/2018 03:00 PM, Igor Mammedov wrote:
> [...]
> >>>
> >>> I think Igor wants one contiguous region for RAM, where additional
> >>> space can be reserved for hotplugging.    
> >> This is not compliant with 2012 ARM white paper, although I don't really
> >> know if this document truly is a reference (did not get any reply).  
> > it's upto QEMU to pick layout, if we have maxmem (upto 256Gb) we could
> > accommodate legacy req and put single device_memory in 1Gb-256Gb GPA gap,
> > if it's more we can move whole device_memory to 2Tb, 8Tb ...
> > that keeps things manageable for us and fits specs (if such exist).
> > WE should make selection of the next RAM base deterministic is possible
> > when layout changes due to maxram size or IOVA, so that we won't need
> > to use compat knobs/checks to keep machine migratable.  
> Sorry for the delay. I was out of the office those past weeks.
> 
> OK understood. Your preferred approach is to have a contiguous memory
> region (initial + hotplug). So this depends on the FW capability to
> support flexible RAM base. Let's see how this dependency gets resolved.
I think Drew had already a look at FW side of the issue and has
a prototype to works with.
Once he's back in the office he planned to work on upstreaming EDK
and qemu parts.
 
> This series does not bump the non hotpluggable memory region limit,
> which is still limited to 255GB. The only way to add more memory is
> though PCDIMM or NVDIMM (max 2TB atm). To do so you need to add ,maxmem
> and ,slots options which need to be on both source and dest, right, +
> the PCDIMM/NVDIMM device option lines? Also the series checks the
> destination has at least the same IPA range capability as the source,
> which conditions the fact the requested device_memory size can be
> accommodated. At the moment I fail to see what are the other compat
> knobs I must be prepared to handle.
it looks the same to me.

We might use presence of slot/maxmem options as a knob to switch
to a new all DIMM layout (initial + hotplug) with floating ram base.
That way guests/fw that are designed to work with fixed RAM base will
work just fine by default and guests/fw that are to work with
mem hotplug or large RAM need vfio holes will use floating RAM base.
Does it seem reasonable?


> Thanks
> 
> Eric
> > 
> > [...]
> >