Re: [PATCH 1/2] kvm tools: Add memory gap for larger RAM sizes

[Sasha Levin <levinsasha928@gmail.com>]

On Wed, 2011-05-11 at 08:37 +0300, Pekka Enberg wrote:
> On Wed, May 11, 2011 at 12:05 AM, Sasha Levin <levinsasha928@gmail.com> wrote:
> > +       if (kvm->ram_size < 0xe0000000) {
> 
> Please use the ULL postfix for constants to ensure the types are sane.
> Also, please come up with a sane name for these.
> 
> > @@ -60,7 +60,14 @@ static inline u32 segment_to_flat(u16 selector, u16 offset)
> >
> >  static inline void *guest_flat_to_host(struct kvm *self, unsigned long offset)
> >  {
> > -       return self->ram_start + offset;
> > +       /*
> > +        * We have a gap between 0xe0000000 and 0x100000000.
> > +        * Consider it when translating an address above 0x100000000.
> > +        */
> > +       if (offset < 0xe0000000)
> > +               return self->ram_start + offset;
> > +       else
> > +               return self->ram_start + 0xe0000000 + (offset - 0x100000000);
> >  }
> 
> Would it not be simpler to mmap() a "ram_size + gap_size" contiguous
> region and mprotect(PROT_NONE) the gap? We'd still tell KVM and E820
> maps about two separate regions but guest_flat_to_host() would
> work-as-is.

I've wanted to avoid actually allocating that gap (which is currently
512MB) and instead take the hit in guest_flat_to_host().

If you feel the 512MB vs guest_flat_to_host() trade-off is worth it,
I'll change it to work that way.

-- 

Sasha.