Re: [Qemu-devel] [PATCH v4 2/5] x86: Allow physical address bits to be set

[Eduardo Habkost]

On Fri, Jul 08, 2016 at 04:01:36PM +0100, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
> 
> Currently QEMU sets the x86 number of physical address bits to the
> magic number 40.  This is only correct on some small AMD systems;
> Intel systems tend to have 36, 39, 46 bits, and large AMD systems
> tend to have 48.
> 
> Having the value different from your actual hardware is detectable
> by the guest and in principal can cause problems;
> The current limit of 40 stops TB VMs being created by those lucky
> enough to have that much.
> 
> This patch lets you set the physical bits by a cpu property but
> defaults to the same 40bits which matches TCGs setup.
> 
> I've removed the ancient warning about the 42 bit limit in exec.c;
> I can't find that limit in there and no one else seems to know where
> it is.

I think I've found it, buried in ancient commits:

  commit 5270589032f450ae7c3448730855aa18ff68ccff
  Author: Richard Henderson <address@hidden>
  Date:   Wed Mar 10 14:33:23 2010 -0800
  
      Move TARGET_PHYS_ADDR_SPACE_BITS to target-*/cpu.h.
      
      Removes a set of ifdefs from exec.c.
      
      Introduce TARGET_VIRT_ADDR_SPACE_BITS for all targets other
      than Alpha.  This will be used for page_find_alloc, which is
      supposed to be using virtual addresses in the first place.
      
      Signed-off-by: Richard Henderson <address@hidden>
[...]
  --- a/exec.c
  +++ b/exec.c
  [...]
  -#elif defined(TARGET_X86_64)
  -#define TARGET_PHYS_ADDR_SPACE_BITS 42
  -#elif defined(TARGET_I386)
  -#define TARGET_PHYS_ADDR_SPACE_BITS 36
[...]
  --- a/target-i386/cpu.h
  +++ b/target-i386/cpu.h
[...]
  +#ifdef TARGET_X86_64
  +#define TARGET_PHYS_ADDR_SPACE_BITS 52
  +/* ??? This is really 48 bits, sign-extended, but the only thing
  +   accessible to userland with bit 48 set is the VSYSCALL, and that
  +   is handled via other mechanisms.  */
  +#define TARGET_VIRT_ADDR_SPACE_BITS 47
  +#else
  +#define TARGET_PHYS_ADDR_SPACE_BITS 36
  +#define TARGET_VIRT_ADDR_SPACE_BITS 32
  +#endif
  +

But I really don't understand why it was changed to 52 when the
macros were moved. A (fortunate) typo?

-- 
Eduardo