[RFC PATCH v2 14/32] x86: mm: Provide support to use memblock when spliting large pages

[Borislav Petkov]

On Thu, Mar 16, 2017 at 11:25:36PM +0100, Paolo Bonzini wrote:
> The kvmclock memory is initially zero so there is no need for the
> hypervisor to allocate anything; the point of these patches is just to
> access the data in a natural way from Linux source code.

I realize that.

> I also don't really like the patch as is (plus it fails modpost), but
> IMO reusing __change_page_attr and __split_large_page is the right thing
> to do.

Right, so teaching pageattr.c about memblock could theoretically come
around and bite us later when a page allocated with memblock gets freed
with free_page().

And looking at this more, we have all this kernel pagetable preparation
code down the init_mem_mapping() call and the pagetable setup in
arch/x86/mm/init_{32,64}.c

And that code even does some basic page splitting. Oh and it uses
alloc_low_pages() which knows whether to do memblock reservation or the
common __get_free_pages() when slabs are up.

So what would be much cleaner, IMHO, is if one would reuse that code to
change init_mm.pgd mappings early without copying pageattr.c.

init_mem_mapping() gets called before kvm_guest_init() in setup_arch()
so the guest would simply fixup its pagetable right there.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--