Re: [PATCH DOCDAY] xen: write a high level description of the sub-arch choices for heap layout

[Ian Campbell <ian.campbell@citrix.com>]

On Wed, 2015-09-30 at 04:33 -0600, Jan Beulich wrote:
> > > > On 30.09.15 at 12:22, <ian.campbell@citrix.com> wrote:
> > The 3 options which (sub)arches have for the layout of their heaps is
> > a little subtle (in particular the two CONFIG_SEPARATE_XENHEAP=n
> > submodes) and can be a bit tricky to derive from the code.
> > 
> > Therefore try and write down some guidance on what the various modes
> > are.
> > 
> > Note that this is intended more as a high level overview rather than a
> > detailed guide to the full page allocator interfaces.
> 
> Thanks for writing this up, just two minor things:
> 
> > + * CONFIG_SEPARATE_XENHEAP=n W/ DIRECT MAP OF ALL RAM
> > + *
> > + *   All of RAM is covered by a permanent contiguous mapping and there
> > + *   is only a single heap.
> > + *
> > + *   Memory allocated from the Xen heap is flagged (in
> > + *   page_info.count_info) with PGC_xen_heap which may be used to
> > + *   check and enforce correct usage of va()/pa() etc. Memory
> 
> What is this "check and enforce" about? There are validation uses
> of the flag, but I don't recall any in virt<->phys address translation.

I think I misremembered/assumed.

Arm has an is_xen_heap_page check for the separate xenheap case.

I suppose s/may be/could be/ would be true (sort of) but I guess I'll just
drop that bit.

> > + *   allocated from the Dom heap must still be explicitly mapped
> > + *   before use (e.g. with map_domain_page) in particular in common
> > + *   code.
> > + *
> > + *   xenheap_max_mfn() should not be called by arch code.
> > + *
> > + *   This mode of operation is most commonly used by 64-bit arches
> > + *   which have sufficient free virtual address space to permanently
> > + *   map the largest practical amount RAM currently expected on that
> > + *   arch.
> > + *
> > + * CONFIG_SEPARATE_XENHEAP=n W/ ONLY DIRECT MAP OF ONLY PARTIAL RAM
> 
> I think one of the two ONLY should be dropped.

Agreed.

Ian.