Re: [Xen-devel] [PATCH 4/4] hvmloader: add support to load extra ACPI tables from qemu

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Tue, Jan 26, 2016 at 01:58:35PM +0000, George Dunlap wrote:
> On 26/01/16 12:44, Jan Beulich wrote:
> >>>> On 26.01.16 at 12:44, <George.Dunlap@xxxxxxxxxxxxx> wrote:
> >> On Thu, Jan 21, 2016 at 2:52 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> >>>>>> On 21.01.16 at 15:01, <haozhong.zhang@xxxxxxxxx> wrote:
> >>>> On 01/21/16 03:25, Jan Beulich wrote:
> >>>>>>>> On 21.01.16 at 10:10, <guangrong.xiao@xxxxxxxxxxxxxxx> wrote:
> >>>>>> c) hypervisor should mange PMEM resource pool and partition it to 
> >>>>>> multiple
> >>>>>>     VMs.
> >>>>>
> >>>>> Yes.
> >>>>>
> >>>>
> >>>> But I Still do not quite understand this part: why must pmem resource
> >>>> management and partition be done in hypervisor?
> >>>
> >>> Because that's where memory management belongs. And PMEM,
> >>> other than PBLK, is just another form of RAM.
> >>
> >> I haven't looked more deeply into the details of this, but this
> >> argument doesn't seem right to me.
> >>
> >> Normal RAM in Xen is what might be called "fungible" -- at boot, all
> >> RAM is zeroed, and it basically doesn't matter at all what RAM is
> >> given to what guest.  (There are restrictions of course: lowmem for
> >> DMA, contiguous superpages, &c; but within those groups, it doesn't
> >> matter *which* bit of lowmem you get, as long as you get enough to do
> >> your job.)  If you reboot your guest or hand RAM back to the
> >> hypervisor, you assume that everything in it will disappear.  When you
> >> ask for RAM, you can request some parameters that it will have
> >> (lowmem, on a specific node, &c), but you can't request a specific
> >> page that you had before.
> >>
> >> This is not the case for PMEM.  The whole point of PMEM (correct me if
> >> I'm wrong) is to be used for long-term storage that survives over
> >> reboot.  It matters very much that a guest be given the same PRAM
> >> after the host is rebooted that it was given before.  It doesn't make
> >> any sense to manage it the way Xen currently manages RAM (i.e., that
> >> you request a page and get whatever Xen happens to give you).
> > 
> > Interesting. This isn't the usage model I have been thinking about
> > so far. Having just gone back to the original 0/4 mail, I'm afraid
> > we're really left guessing, and you guessed differently than I did.
> > My understanding of the intentions of PMEM so far was that this
> > is a high-capacity, slower than DRAM but much faster than e.g.
> > swapping to disk alternative to normal RAM. I.e. the persistent
> > aspect of it wouldn't matter at all in this case (other than for PBLK,
> > obviously).
> 
> Oh, right -- yes, if the usage model of PRAM is just "cheap slow RAM",
> then you're right -- it is just another form of RAM, that should be
> treated no differently than say, lowmem: a fungible resource that can be
> requested by setting a flag.

I would think of it as MMIO ranges than RAM. Yes it is behind an MMC - but
there are subtle things such as the new instructions - pcommit, clfushopt,
and other that impact it.

Furthermore ranges (contingous and most likely discontingous)
of this  "RAM" has to be shared with guests (at least dom0)
and with other (multiple HVM guests).


> 
> Haozhong?
> 
>  -George
> 
> 
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