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

[igor Mammedov <imammedo@redhat.com>]

On Fri, 25 Oct 2013 11:34:22 -0200
Marcelo Tosatti <mtosatti@redhat.com> wrote:

> On Fri, Oct 25, 2013 at 11:57:18AM +0200, igor Mammedov wrote:
> > On Fri, 25 Oct 2013 02:58:05 -0200
> > Marcelo Tosatti <mtosatti@redhat.com> 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)]
> 
> Yes, thought of that, unfortunately its cumbersome to add an interface
> for the user to supply both 2MB and 1GB hugetlbfs pages.
Could 2Mb tails be automated, meaning if host uses 1Gb hugepages and
there is/are tail/s, QEMU should be able to figure out alignment
issues and allocate with appropriate pages.

Goal is separate host part allocation aspect from guest related one,
aliasing 32-bit hole size at the end doesn't help it at all, it's quite
opposite, it's making current code more complicated and harder to fix
in the future.