Re: [Xen-devel] [PATCH 3/4] xen: arm: rearrange guest physical address space to increase max RAM

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-04-08 at 16:12 +0100, Julien Grall wrote:
> Hi Ian,
> 
> On 04/08/2014 03:19 PM, Ian Campbell wrote:
> > By switching things around we can manage to expose up to 3GB of RAM to 
> > guests.
> > 
> > I deliberately didn't place the RAM at address 0 to avoid coming to rely on
> > this, so the various peripherals, MMIO and magic pages etc all live in the
> > lower 1GB leaving the upper 3GB available for RAM.
> > 
> > It would likely have been possible to reduce the space used by the 
> > peripherals
> > etc and allow for 3.5 or 3.75GB but I decided to keep things simple and will
> > handle >3GB memory in a subsequent patch.
> > 
> > Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > ---
> >  xen/include/public/arch-arm.h |   18 +++++++++---------
> >  1 file changed, 9 insertions(+), 9 deletions(-)
> > 
> > diff --git a/xen/include/public/arch-arm.h b/xen/include/public/arch-arm.h
> > index b860da5..5840453 100644
> > --- a/xen/include/public/arch-arm.h
> > +++ b/xen/include/public/arch-arm.h
> > @@ -364,18 +364,18 @@ typedef uint64_t xen_callback_t;
> >   */
> >  
> >  /* Physical Address Space */
> > -#define GUEST_GICD_BASE   0x2c001000ULL
> > -#define GUEST_GICD_SIZE   0x1000ULL
> > -#define GUEST_GICC_BASE   0x2c002000ULL
> > -#define GUEST_GICC_SIZE   0x100ULL
> > +#define GUEST_GICD_BASE   0x03001000ULL
> > +#define GUEST_GICD_SIZE   0x00001000ULL
> > +#define GUEST_GICC_BASE   0x03002000ULL
> > +#define GUEST_GICC_SIZE   0x00000100ULL
> >  
> > -#define GUEST_RAM_BASE    0x80000000ULL /* 768M at 2GB*/
> > -#define GUEST_RAM_END     0xafffffffULL
> > -
> > -#define GUEST_GNTTAB_BASE 0xb0000000ULL
> > +#define GUEST_GNTTAB_BASE 0x38000000ULL
> >  #define GUEST_GNTTAB_SIZE 0x00020000ULL
> 
> Not related to this patch... while you are re-working the guest layout.
> Can you comment where does come from the GNTTAB_SIZE...?

Stefano added that one, I assume he made it up...

> Also, can you make sure that the GNTTAB_SIZE is greater or equal to the
> maximum number of frames (maybe by overriding max_nr_grant_frames)?

It turns out that the current size corresponds to
DEFAULT_MAX_NR_GRANT_FRAMES, which explains where it came from.

If you want to change this to reserve say 1MB of address space (which is
enough for 256 grant pages) or even more then please send a patch.

> The current implementation on Linux only care about the number of frames
> given by Xen, the size of the table in the DT is not used. So the range
> may overlap to something else.

That would be a guest bug, but nothing to do with this series.

Ian.


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