Re: [Xen-devel] [PATCH v2 16/17] libxc/xc_dom_arm: Copy ACPI tables to guest space

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Jul 28, 2016 at 08:42:05PM +0800, Shannon Zhao wrote:
> On 2016年07月28日 19:06, Julien Grall wrote:
> > On 26/07/16 02:17, Boris Ostrovsky wrote:
> >> On 07/25/2016 07:40 PM, Stefano Stabellini wrote:
> >>> On Mon, 25 Jul 2016, Boris Ostrovsky wrote:
> >>>> On 07/25/2016 06:06 PM, Stefano Stabellini wrote:
> >>>>> On Mon, 25 Jul 2016, George Dunlap wrote:
> >>>>>> On Thu, Jul 21, 2016 at 10:15 PM, Stefano Stabellini
> >>>>>> <sstabellini@xxxxxxxxxx> wrote:
> >>>>> Going back to the discussion about how to account for the ACPI blob in
> >>>>> maxmem, let's make this simple, if we increase maxmem by the size of
> >>>>> the
> >>>>> ACPI blob:
> >>>>>
> >>>>> - the toolstack allocates more RAM than expected (bad)
> >>>>> - when the admin specifies 1GB of RAM, the guest actually gets 1GB of
> >>>>>    usable RAM (good)
> >>>>> - things are faster as Xen and the guest can exploit superpage
> >>>>> mappings
> >>>>>    more easily at stage-1 and stage-2 (good)
> >>>>>
> >>>>> Let's call this option A.
> >>>>>
> >>>>> If we do not increase maxmem:
> >>>>>
> >>>>> - the toolstack allocates less RAM, closer to the size specified in
> >>>>> the
> >>>>>    VM config file (good)
> >>>>> - the guest gets less usable memory than expected, less than what was
> >>>>>    specified in the VM config file (bad)
> >>>>
> >>>> Not sure I agree with this, at least for x86/Linux: guest gets 1GB of
> >>>> usable RAM and part of that RAM stores ACPI stuff. Guest is free to
> >>>> stash ACPI tables somewhere else or ignore them altogether and use that
> >>>> memory for whatever it wants.
> >>> On ARM it will be a ROM (from guest POV)
> >>
> >>
> >> In which case I don't see why we should take it from maxmem allocation.
> >> I somehow thought that there was a choice of whether to put it in ROM or
> >> RAM on ARM but if it's ROM only then I don't think there is an option.
> > 
> > We have option to do the both on ARM. I just feel that the ROM option is
> > a cleaner interface because the ACPI tables are not supposed be modified
> > by the guest, so we can prevent to be overridden (+ all the advantages
> > mentioned by Stefano with option A).
> > 
> >> IIUIC the toolstack pretends that the blob goes to memory because that's
> >> how its interfaces work but that space is not really what we think about
> >> when we set memory/maxmem in the configuration file. Unlike x86.
> > 
> > I think we need to draw a conclusion for Shannon to continue to do the
> > work and I would like to see this series in Xen 4.8. From my
> > understanding you are for option B, so does George.
> > 
> > Stefano votes for option A, but find B acceptable. Any other opinions?
> I agree with Stefano, both are fine.
> 

Sorry for the late reply.

Are you now unblocked? If not, what is not yet decided or needed
clarification?

Wei.

> Thanks,
> -- 
> Shannon

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel