Re: [Qemu-devel] [patch 2/2] i386: pc: align gpa<->hpa on 1GB boundary

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Oct 25, 2013 at 12:55:36AM +0100, Paolo Bonzini wrote:
> > +        if (hpagesize == (1<<30)) {
> > +            unsigned long holesize = 0x100000000ULL - below_4g_mem_size;
> > +
> > +            memory_region_init_alias(ram_above_4g, NULL, "ram-above-4g", ram,
> > +                                    0x100000000ULL,
> > +                                    above_4g_mem_size - holesize);
> > +            memory_region_add_subregion(system_memory, 0x100000000ULL,
> > +                                    ram_above_4g);
> > +
> > +            ram_above_4g_piecetwo = g_malloc(sizeof(*ram_above_4g_piecetwo));
> > +            memory_region_init_alias(ram_above_4g_piecetwo, NULL,
> > +                                     "ram-above-4g-piecetwo", ram,
> > +                                     0x100000000ULL - holesize, holesize);
> > +            memory_region_add_subregion(system_memory,
> > +                                        0x100000000ULL +
> > +                                        above_4g_mem_size - holesize,
> > +                                        ram_above_4g_piecetwo);
> 
> Why break it in two?  You can just allocate extra holesize bytes in the
> "ram" MemoryRegion, and not map the part that corresponds to
> [0x100000000ULL - holesize, 0x100000000ULL).

- If the "ram" MemoryRegion is backed with 1GB hugepages, you might not 
want to allocate extra holesize bytes (which might require an entire
1GB page).

- 1GB backed RAM can be mapped with 2MB pages.

> Also, as Peter said this cannot depend on host considerations.  Just do
> it unconditionally, but only for new machine types (pc-1.8 and q35-1.8,
> since unfortunately we're too close to hard freeze).

Why the description of memory subregions and aliases are part of machine
types?