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

[igor Mammedov]

On Fri, 25 Oct 2013 02:58:05 -0200
Marcelo Tosatti <address@hidden> wrote:

> 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).
From POV of moddeling current "ram" as dimm devices, aliasing
wouldn't work nice. But breaking one block in two or more is fine since
then blocks could be represented as several dimm devices.

+3Gb backend ram it could be split in blocks like this:

  [ 3Gb (1Gb pages backed) ]
  [tail1 (below_4gb - 3Gb) (2mb pages backed) ]
  [above_4gb whole X Gb pages (1Gb pages backed)]
  [tail2 (2mb pages backed)]

> - 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?
> 
>