Re: [Xen-devel] QEMU bumping memory bug analysis

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Fri, 2015-06-05 at 18:10 +0100, Stefano Stabellini wrote:
> On Fri, 5 Jun 2015, Wei Liu wrote:
> > Hi all
> > 
> > This bug is now considered a blocker for 4.6 release.
> > 
> > The premises of the problem remain the same (George's translated
> > version):
> > 
> > 1. QEMU may need extra pages from Xen to implement option ROMS, and so at
> >    the moment it calls set_max_mem() to increase max_pages so that it can
> >    allocate more pages to the guest.  libxl doesn't know what max_pages a
> >    domain needs prior to qemu start-up.
> > 
> > 2. Libxl doesn't know max_pages even after qemu start-up, because there
> >    is no mechanism to communicate between qemu and libxl.
> 
> I might not know what is the right design for the overall solution, but
> I do know that libxl shouldn't have its own state tracking for
> max_pages, because max_pages is kept, maintained and enforced by Xen.
> 
> Ian might still remember, but at the beginning of the xl/libxl project,
> we had few simple design principles. One of which was that we should not
> have two places where we keep track of the same thing. If Xen keeps
> track of something, libxl should avoid it.

This isn't about libxl tracking something duplicating information in
Xen. It is about who gets to choose what that value is, which is not the
same as who stores that value.

So this is about libxl being the owner of what the current maxmem value
is. It can so this by using setmaxmem and getmaxmem to set and retrieve
the value with no state in libxl.

> I disagree that libxl should be the arbitrator of a property that is
> stored, maintained and enforced by Xen. Xen should be the arbitrator.

That's not what "arbitrator" means, an arbitrator decides what the value
should be, but that doesn't necessarily imply that it either stores,
maintains nor enforces that value. 

> Even if QEMU called into libxl to change maxmem, I don't think that
> libxl should store maxmem anywhere. It is already stored in Xen.

I don't think anyone suggested otherwise, did they?

What locking is there around QEMU's read/modify/write of the maxmem
value? What happens if someone else also modifies maxmem at the same
time?

Ian.


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